2fa/new device question

Is the phone number of the apple id's device(s) considered a "trusted number"? If so, what's stopping someone who had access to the account before 2fa was added from just selecting the SMS options and getting in that way, since they would technically still have access to the phone number.

where would the 2fa confirmation be then? would it be on any/all of the other devices or just one?

dont all devices with the same apple id use the same phone number?

i swear my old iphone under my apple id account has the same phone number. i got all the data from my old iphone transferred to a newer one, which i currently use now, maybe thats why?

i don't know how the transferring went down, but it was originally my mothers phone. im assuming it was just wiped clean and then all my data was transferred to it.