An elderly relative of mine was hit by this a couple years back: his computer's desktop was constantly being spammed with messages on startup, and there was no simple way to turn them all off. It turned out that they were all notifications from web workers that he'd inadvertently allowed at some point prior. (I set his browser to auto-deny notifications so it wouldn't happen again.)
What's worse is they look like native OS alerts (on Windows) so when one says "SECURYIRT ALERT!! CALL NOW" it's that much more effective at getting people on the phone with scammers.
Good overviews of the truck https://youtu.be/aEq-vTLimrQ?si=fS-UhjndoWuxwBip
https://youtu.be/1OgN_qctcGs?si=nEysWQHzafRpxfRp