I'm pretty sure Facebook does simple IP address tracking to serve ads. I've noticed my wife and I both get served ads for things the other has recently searched for.
I was reading an news article on my phone on NYT just yesterday while someone else in the house was buying things from a big online clothing store (not amazon) on her phone at the other end of the couch. In real time the ad slots on the article on my phone switched to show ads for the site she was buying from and for similar products. The had previously been showing me something i bought a few days ago.
The reverse doesn't happen: my DDG searches and FF browsing never bleed on Youtube.
Google (and many others) doesn't care about private mode, separate accounts, profiles or containers.
Every single ISP available to me advertises it as a feature: “get fixed IP for free when you use IPv6 (usually $5/month for IPv4)”. When I asked if there’s non-fixed IPv6 they were perplexed at my request and said no.