Well yes, logging in via another app, espciallly FB is v dangerous, Many apps use this, including ones like My Fitness Pal and Fitbit where you willingly give away medical information.
And even if you don't log in using FB, you can be linked by name and a digital fingerprint.
Use tracker blockers, avoid apps as much a possible on your phone and don't use your real name unless you have to. Apss collect all manner of information from your phobe, even when it is not necessary for the functionality of the app. Check the permissions before you dowmload. As a rule, pay for the app rather than using a free one - your data is not free!
And use a VPN for super safety. And get off FB, it's evil
I have been banging on about on here this for years but kept being called paranoid, so I gave up warning people
And don't get me started on Elexa and google