Yeah with that they're likely talking about apps spying on you. That isn't really a phone thing, but pinning down exactly which apps do it is hard. Almost certainly it's Facebook apps (especially the hidden system apps they get manufacturers to install), wouldn't be surprised if reddit's app does some of that as well.

Personally, I've not had that problem in years, but I run custom ROMs without gapps, and generally try to stick with FOSS apps. However it definitely is a thing where you have a conversation about something around your phone and then soon after see ads for that thing.