By definition, discussion mailing lists are intended to foster discussion amongst the subscribers. For this reason, typical use means replies are directed back to the entire list instead of to just the original author.

Your CCtoMany list manager does this by inserting a "Reply-To" header to point to the list address instead of the original author. Thus regardless of the sender, when the recipient of a list message hits reply, that reply goes to the entire list by default.

Recipients can always edit the To: header on their reply to address it only to the original sender if they wish, but few will do that.

NOTE: We can instead configure a list so that "Reply-to" includes only the original author and NOT the entire list. (In that case, "Reply-to-all" will generally include the original author AND the full discussion list. This is dependent on individual email client behaviour but most operate in that way.)