Most of the assets in any IRA are themselves taxed. For example, if you own any stock in an IRA, those companies pay income tax on their earnings. The tax-free element comes in that when you receive dividends, you aren’t taxed on them. Likewise, when you sell the assets, no capital gain tax is recognized. Investment-type income is what receives the special tax-deferred gains.
An LLC can be taxed as a pass-through entity, though. Doing that would meant that the IRA might have unrelated business income. For example, if the LLC manufactures widgets, the widget income is unrelated business income. There is a tax to the IRA on the unrelated income.
Anyway, the difference between an LLC and SDLLC is that the latter is owned by an IRA. In fact, I would say that the term SDLLC is misleading - it’s a self-directed IRA that happens to own an LLC as its assets. The LLC is no different than any other. it’s just that its owner is an IRA rather than a person.
There’s no change in how much money you can put into the IRA. Those limits are set regardless of what assets the IRA holds The real point is to use money you already have in an IRA through the company, since you wouldn’t want to pay tax and penalties by withdrawing money from an IRA to use in an outside LLC.
I don’t know if the LLC can get around the limits usually applied to the kinds of assets owned by an IRA. It seems to me that it probably could. However, you come back to paying unrelated business income tax on non-qualified kinds of income/assets. It definitely doesn’t let you get around the rules against benefiting from the investments - you couldn’t buy your car or house through the LLC/IRA.