I’ve been thinking about this.
I am thinking essentially the solution is the equivalent of multi reddits. In the UI, either from the user perspective, or the server admin perspective, you can setup communities that are just aggregations of different communities.
Then a user can choose to browse /c/cats which is actually just /c/cats from lemmy.world, lemmy.ml, etc as one feed.
I worked at Tinder, we had something like 100 engineers for 20 million or whatever daily active users., and I think it was rather well managed with everyone doing a part. Reddit is 20x user wise and far more complex feature wise, so maybe it makes sense.
It seems absurd, but there’s a lot of things going on that you don’t think about. Bots, Ads, Moderation tooling, User management, Chat feature, NFTs, revenue features, push notifications, user targeting, ranking algorithms, etc all consist of whole teams.