So long as they have the same email address, you can sign in with any of them.
For example, if you initially signed up with email but then sign in again with an Apple account that uses the same email address, you will be signed in to the same account. The email and Apple sign-ins will become linked.
You can see which sign-in method you used at the bottom of the Account screen.
For example in the above screenshot, I’ve signed in using both email and Google on different occasions, so the sign-ins are now linked, and it says “Signed in with email and Google”.