A number of things:
When you eventually retire, you should rollover all of your 401k assets from your employer’s plan into an/some IRA(s), so that you have complete control over the funds. If you have designated part of your salary deferrals as Roth contributions, then those assets would clearly be best rolled over into a Roth IRA, while everything else could go into a traditional IRA. You could rollover everything into the same IRA, but then you’d either have to pay lots of tax (if into a Roth) or keep track of your basis in your IRA (if into traditional) which I can tell you is absolutely no fun and is not as good tax-wise as keeping the Roth funds in a Roth account.
It’s probably too late for you to make very good use of a Roth 401k feature, since right before you retire from your main job is when you typically make the most money and you’ll typically be in the highest tax bracket you will ever be in. The main reason to designate some funds as Roth contributions is that you expect to pay less in taxes now than when you plan to take the funds out of the IRA, so all you really care about is relative tax rates between now and retirement. There also is the issue of tax rates possibly changing, but that’s hard to predict. Another possible use for Roth designated funds is to have a source of income in retirement that’s not taxable which allows you to take out extra funds as needed for things without incurring tax at a higher bracket than normal. But if you don’t have a decent chunk of change in there already, it’s probably not going to be too helpful.
In order for the earnings of your Roth designated funds to not be taxed on withdrawal, you have to wait 5 years after opening the account. You can always withdral your after-tax contributions in Roth IRAs with no tax or penalty, although I’m not so sure how it works with respect to Roth 401k that’s rolled over into a Roth IRA. I assume your 401k custodian keeps track of your basis in the Roth 401k, but I have no personal experience. This means that you can withdraw most of it basically as soon as your put it in, but if you were to make an awesome stock pick and have your account quintuple in value, you would only be able to withdraw one fifth of it tax-free until 5 years, regardless of how old you are. It’s never a good idea to take out your contributions from a Roth plan before retirement because you can’t put them back in later, but you can at least do it in a pinch and not technically pay taxes on it (you just lose the benefit of the tax-free earnings you would have gotten had you left it in). Once you hit 59.5 and five years of an active account, all your Roth distributions are tax-free. However, sometimes your custodian will not know for sure whether you meet the requirements, and code your 1099-R such that software will by default make it taxable; in such a case a good tax preparer/preparation software will ask you the necessary questions to determine whether it is taxable, but bad preparers who ignore their software’s diagnostics will just accept it’s taxable without further thought because they are pressed for time and care more about cranking out returns than getting them right.
Most people who will be withdrawing from Roth funds in the near future had those funds contributed directly into a Roth IRA in addition to maxing out their 401k, because as mentioned above it doesn’t make sense for people later in their career to be using the Roth 401k option. For a Roth IRA contribution to be made, you have to make under a certain amount of money in the tax year that the funds are contributed for. That’s currently around $133k for single and $200k for married filing jointly, but those are taken off the top of my head. This is a very good deal for those that would contribute more to their 401ks but can’t. You need earned income to make IRA contributions, but you can continue making Roth contributions your entire life so long as you have earned income, and you never need to make withdrawals during your lifetime (your heirs will though, assuming there’s something left). The limit for the 2019 tax year is $6,000 + $1,000 if over 50. This is up from 2018 and caused me some confusion during tax season this year since I contributed $6,000 to my Roth IRA fairly near the beginning of the year, but was seeing clients say they were making $5,500 contributions. It wasn’t until I realized that they were doing this for the previous tax year, not wanting to commit the funds until their accountant said it was OK, that it made sense. People make poor decisions when it comes to making IRA contributions all the time because they aren’t aware of the restrictions, so it makes sense for them to wait for their tax return to be finished before putting the money in.
By making Roth designated contributions, you effectively get to put more into your retirement plan than making all traditional contributions, since the dollar amount is the same but it’s made with post-tax money, which is worth more than pre-tax money. So despite what I said above about it being better tax-wise to do all traditional contributions if your tax rate is going to be lower in retirement, that assumes that you actually contribute less to the Roth account with the difference going to pay the taxes on the funds used to make Roth contributions. So it can make sense to switch to all Roth contributions so long as you’re willing to take a reduced paycheck because the federal income tax withheld will go up, and are willing to wait long enough to withdraw the funds such that the additional tax paid on putting them into the Roth account is outweighed by the additional tax-free earnings the account generates. What that time frame is depends on how the market does and what the difference in tax rates is. It’s impossible to say in general. So going to Roth contributions does effectively get more money into retirement accounts when you’re up against the limit, but might not be worth the extra tax you pay upfront.
A backdoor Roth conversion is where you make too much money to make a Roth IRA contribution. Instead, you make a nondeductible traditional IRA contribution, which can always be made, and then roll over the funds over into a Roth immediately. The only hoop you have to jump through in order to do this is have absolutely no other money in a traditional IRA. For people that have changed employers and rolled the 401k funds into IRAs so they could get the funds out of the previous employer’s control, they basically can’t do this. What happens is that Roth rollovers from traditional plans are taxable distributions, but the taxable amount is reduced by an amount of your “basis” in your traditional IRA that is allocable to the amount you rolled over. People generally don’t have any basis in IRAs rolled over from 401k plans; the only way you get basis is by making nondeductible traditional IRA contributions*. If you have multiple traditional IRAs and have some basis in them, when you rollover part of your IRA into a Roth (or simply withdraw it normally), the percentage that is tax free is the same as the percentage your basis is as a whole of all your IRAs. If you have a $50,000 IRA and make a $5,000 “backdoor” contribution and Roth rollover, you actually only get to use $500 of your basis, and leave $4500 in basis for later withdrawals and being taxed now on $4500 of the rollover. There is actually a client of ours who is doing something like this. We’re not their financial planner, we just do their tax return, and their decisions on how to fund their retirement plan are their own. If they were my direct client I would advise them to stop, but I’m just the peon who enters data for that particular person. There are multiple other clients who are doing it the “right” way - they have no other money in an IRA.
*On general accounting principles, it may be that you can rollover a Roth IRA into a traditional IRA and then you’d have basis in your rollover IRA. But this is absolutely a terrible idea, and it’s possible that you’re not even allowed to do it because of how colossally stupid it would be. It’s so stupid that I haven’t even entertained the idea before and would have no reason to look up if it’s actually possible. The only reason people make nondeductible contributions to traditional IRAs is because they want money in a retirement account and have no other way of getting it there. Nondeductible traditional IRA contributions are better than leaving the money in a taxable account, but just barely compared to deductible traditional contributions or Roth contributions, which are effectively the same if your marginal tax rate doesn’t change. The only thing that would make it a good idea is if you expect the market value of your account to go down due to market fluctuations and you have other traditional IRAs, but if that were the case, you could just put the funds into cash and let them sit there while the market falls. Since markets tend to go up, and that’s why you invest, making nondeductible traditional IRA contributions or rollovers should be a last resort.