Subscribe button

It sits under the player on a watch page, on the channel page, in the Shorts overlay and inside end screens. Clicking it adds the channel to that viewer's subscriptions feed, and choosing notifications on top of that puts new uploads in their alerts. Those are two separate decisions, and YouTube stops pushing a channel's notifications to anyone who has gone roughly a month without watching it or engaging with its alerts, routing the upload to their inbox instead. So a large subscriber count and a large notified audience are different things.