Thanks, that's an interesting observation! When I double-clicked on the "Siri" process in Activity Monitor, I saw that it's an app called "Siri AI".

If the "Siri AI" app has replaced Spotlight, then I wonder if Apple keeps building "personal context" despite Siri and Apple Intelligence being disabled.

It for sure doesn't sound well to disable Siri and still see a "Siri" process.

This sounds similar to Microsoft and the start menu. A few years ago I went to extreme lengths to disable cortana (powershell guid uninstall). This invisibly and inexplicably disabled updates to the start menu search feature. The index remained the feature worked, it just never got updated again. No amount of manually reindexing the disk helped. Reinstalling cortana got it working.

That makes sense. Sending SIGKILL to that process causes the spotlight UI in the menu bar to flash.