I don’t ask why. Of course it is, in a manner of speaking. But how did this happen?
My mom and I are discussing our yearly family get together during the summer. She was looking up houses to rent. We continue talking and go into my room, where I open a new tab on my computer to show her something. One of the first ads? Places to rent in the town we were discussing.
Coincidence?
Your two computers presumably share an IP address. Google knows that your IP address was looking up houses in a particular area a moment ago, so it shows houses in that area. It doesn’t know it’s a different computer.
Adding to what Peter_Morris says; it’s a good habit to delete your browsing history, cookies etc. each time you finish a browsing session. You can do this manually or set the browser to remove history automatically each time you close out a session. You probably don’t care to have your mom know about all those porn sites 
Avoid Google (anything related to them) where possible - which is near impossible. Example; if you’re using Chrome browser, don’t. Brave would be a decent alternative.
Install an ad blocker such as uBlock Origin.
The things I look up in Chrome (my work browser) show up in Brave (my personal browser). Maybe because Brave runs off of Chrome?
Or it does know it’s a different computer, owned by the same person or a person with similar interests to the person who looked up the places to stay. Ads for things similar to what I looked up on one device routinely show up on other devices I use.
Are they on the same computer, or the same network? Google could correlate their usage by IP address, as Peter_Morris pointed out earlier.
Have they ever logged into the same Google account? Google could correlate them as used by the same user that way. Even if it just happened one time, unless you’ve killed the cookies from that dual log in on at least one brower, you could have zombie cookies that Google could use to correlate those browsers together.
And Google has huge numbers of other sites - youtube, for instance - that share user info across them.
For that matter, if you use the “sign in using Google” feature on other web sites, any combination of them could link your computers together, even if the other web sites are not related to each other.
I don’t think I’ve ever done this, maybe once in a while under special circumstances, certainly not on any kind of regular basis. I don’t think I’ve suffered on account of my laxity.
Nor will you . . .until you do. A bit like, not needing insurance.
what’s the worst case scenario? Getting ads you are bothered by?
Well, insurance is taken out against a specific thing – protecting your home, car, need for medical care, etc often with specific values against specific events (theft, fire, collision and so on). It’s not taken out on a vague sense that something somewhere might happen someday.
And you have to weigh that vague idea that cookies might cause some kind of problem against the inconvenience of having to sign in to every website every time you visit it, and losing your website preferences, settings, shopping cart, etc.
Yes, Chrome and Brave are on the same computer. I’ve never logged into Chrome with any personal info, email, etc, nor into Brave with anything work-related. So it could be IP related.
Instead of deleting cookies all the time, with all of the downsides you mention, it is better to use an ad blocker that will block the tracking cookies. If you do not want to block ads (or all ads), then some ad blockers can be tuned to just block tracking, but not ads.
Privacy Badger is also a good choice if you want to let through non-tracking ads. By design it doesn’t require setup or tuning, just install it, and it will start blocking tracking related cookies and ads.
Even blocking tracking cookies, there are still other ways you can be tracked, such as browser fingerprinting in which unique features of your browser (operating system, screen size, available fonts, time zone, etc.) are used to distinguish you from other people. Also, of course, are things like being logged in, and by (possibly shared) IP address.
It’s creepy having your device listening all the time, and even keeping a record of what it hears.
I discovered that my phone not only identifies music it hears in the background, it keeps records going back…years? I can tell when we went to a Mexican restaurant by the succession of Hispanic groups on the list for a particular date.*
They insist it’s not listening orherwise…surre.
*including the time last December when we went to a joint that was playing “La Chica Sexy” by Los Tucanes De Tijuana.
After a decade of “OMG, Big Business Is Watching Me!”, and being a bit paranoid, I’ve had to make a conscious decision to only be bothered by Actual Real Evil Things.
So when I encounter Less Evil Things, like looking up info on a town and later being shown hotels there, I am now shrugging and saying “Thanks, Big Tech, but you’re wasting your time with me. I’ll find my own lodging if I ever go there.”
It’s still creepy, but on a scale of Harmless to Death By Ice, it’s in the bottom 10%…
This is my attitude exactly. I have an ad blocker and uBlock Origin for YouTube, so I see very few ads. I’m careful about links that I click on. And I’m the only one who uses a computer in this house (my husband is connected to the internet via a Fire thingie on his TV, there doesn’t seem to be a lot of room for mischief there). If that were to change, I might consider tightening my security precautions.
As for the creepy factor, I’m fairly sensitive to that as well. That’s why I don’t have smart speakers or smart doorbells or smart appliances of any kind.
The whole “listening in” is weird and scary. I recently fixed a portable generator that wasn’t working that I got for free and plan on selling. Did a search of “value of XXX brand YYY model generator” Seems 90% of my Facebook marketplace and Google ads are about generators.
When my wife is planning a trip she uses data on her phone and then makes the reservations on my laptop. More than once when she is looking at hotels in a specific area the prices are higher on her phone than what shows up on my laptop doing a “cold” search of a specific hotel on my laptop which is connected to our home internet.
I also clear my cookies/history every few days, my wife doesn’t.
Browsers should really have a function whereby you can list a set of cookies/websites that should be kept while deleting any not on the list.