In Apple ID > left menu Password & Security > Apps Using Apple ID: section. When I recently updated my Apple ID password a notice popped up showing that I allow two apps (DropBox and PBS Video) to us my Apple ID. I’d like to learn more about this. If this has already been covered I’d appreciate the link/s or keywords for a search.
The fact that I “unknowingly, to the best of my memory” allow apps to use my Apple ID seems a security concern. Mainly because of my ignorance on the subject.
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Pat Maxim
These are websites and services that use your Apple ID for sign in. When you created the accounts there, instead of manually setting up an account using your email address you clicked a "Sign In with Apple ID" button to set this up. It will quickly set up the account for you and then you can log in quickly too. It is similar to the sign in options that you see offered by Facebook, Google, Twitter and others. But it is more privacy-based.
See https://support.apple.com/en-us/HT210318
Note that these sites and services are not "using your Apple ID." In fact they probably have an email address for your account that is not your Apple ID, but Apple knows to forward email to you. The whole idea here is to allow you to have an account at these sites and services while protecting your privacy and not having to remember and maintain yet another ID+password combination.