According to this video by The Game Theorists, YouTube does not push every video a channel makes to all of that channel's subscriber inboxes. Rather, a subscriber only sees videos from channels they commonly interact with, because that's better marketing.
When I first heard about this I thought the issue was merely a technical limitation. I.e. it would be technologically prohibitive to alert every YouTube user of every new video from every one of their subscriptions. (Considering the massive amounts of data YouTube can transmit this seems unlikely, but who knows.) But the Game Theorists video does not mention technical limitations, just that "YouTube's algorithms are optimized to drive increased watch time".
Is this true? Does YouTube send every video to every subscription inbox or not?
Note that I'm not talking about the YouTube homepage but the subscriptions inbox. I mean, obviously the recommended videos and "updates from channel X" stuff on the homepage are chosen to optimize engagement (and to be honest, it wasn't perfectly clear which one the Game Theorists video was talking about).
Note also that I'm not talking about mobile or google or email notifications. Just about what shows up naturally in the subscription inbox when you click "subscribe".