The current setup is similar to Fathom in that I temporarily track a user session by generating a unique hash for the user, then, if that hash has already been seen in the past 30 mins, we move hash to the latest page view instance and can increment a pages viewed counter for the session. We can't tell which pages you've been on in the past, only that you started a session at X time, and viewed N pages, with the last view at Y time.
Incidentally, I had a PoC for the data ingest running as a Cloudflare Worker using their KV storage. What could be interesting about that is that there'd be zero third-party widget code to inject into a webpage: You'd log the pageview in the worker and pass on the request.
But the market for those Wordpress users who want to paste a few lines of JS snippet into their site would be lost. And it would add a few 100ms to each request you want to log.