They can also be quite smart. What was your referrer, what's your user flow through the site, what do similar users do when arriving from the same pages and searches? What time of day is it for the visitor?

ie: figure out why they are doing what they're doing and direct them to ads that capture that intent.

I'm not certain but much of that sounds like the kind of information GDPR doesn't let you use like that.

The only reason that IP address logging is a problem is that ISPs could theoretically cross reference with subscribers leases and identify them.

I think this much vaguer stuff will be fine.

Why? None of it requires saving info on an individual server-side. As long as you don't do that the GDPR doesn't even apply to you.