YouTube purchases are linked to the account you use to sign in, and are then accessible from any channel owned by that account.
Only the owner of a YouTube channel can make purchases. If you are signed in as a manager of a YouTube channel, but are not the owner, you can't buy a YouTube Premium membership.
When an owner buys a YouTube Premium membership, only they can use the benefits. Brand Account managers will not have access to YouTube Premium membership benefits for a channel that they do not own.