Another great video from Reject Convenience, you should especially watch the part (7:29 - 10:15) about how specific location data can get with precise location on (spoiler alert: a literal battery width radius, please only allow precise location for your map app 🤢)

He also has a great tool he explains throughout the video, his privacy visualizer which you can click as you go through a site's privacy policy to mark what they collect and how they handle the data (just collected, traded/shared, or sold?)

If you want a starting point, try looking at the permissions of the apps you have downloaded. Or try looking at Terms of Service; Didn't Read to figure out who the biggest offenders are first and work your way up to the least creepiest providers.