Apple+

I have multiple Apple devices bought in the last year.

One or more on than one, doesn’t make a difference for offers to Apple TV+.

One offer per user, max, since the 2019 introduction. Apple TV+ offer - Terms and conditions - Apple

Why am I being charged for monthly Apple TV?

If you had a trial for Apple TV+ and that has run its course, then that would transition into a paid subscription after the trial, if you were still subscribed by then.