If your phone is Android they are tracking your location and nearby devices and who owns the devices you are consistently nearby. If your browser is Chrome they know your full browsing history. If your email is Gmail they know everything you emailed anyone or who emailed you.
It is scary how often I'll mention something in a face-to-have conversation and suddenly it's showing up in ads. It's way too often to be coincidence, especially with how unlikely some of those topics are. They are definitely monitoring everything and feeding it into their ad system.
Sometimes I see on YouTube videos "debunking" this but there's never any good evidence either way. I just assume it's Google propaganda.