Rt11111 logging into Play Store caused you to create an entry in the "Password and accounts" sections of your settings. This is basically your hub for accounts and some apps do check and use a fitting account from here (so you don't have to log into all apps from the same company, such as Google).
Basically you have 2 options if you don't like the current setup:
- Remove the Google account and ditch Play Store for something else (good overview in this post)
- Keep the Google account and ditch the other apps for alternatives (I recommend replacing YouTube with Newpipe X Sponsorblock or Libretube)
Maybe I'm wrong here and you can log out of YouTube within the app, I can't test that since I don't have a similar setup...