> one entity that owns the core technology
Correct. Ownership is governed through intellectual property laws, the type of license used, agreements and so on. Lemmy's code is released under a GNU Affero license, and so ownership and usage are governed under this license.
Large open source projects tend to set up legal entities (foundations, non-profits) which manage their brand, trademark, intellectual property rights and so on.
> but the operators of the servers are distributed and independent.
In the era of classic web forums, you would download a copy of vbulletion, phpbb,... operators would download a copy of the software and set up their own intance(s) on their own server(s).
The federated model operates the same way. The distinction, though, is that all these instances are interconnected. They can communicate with each other. In the federated model, you have an account on server A, but you can use that account to subscribe and see content of and participate on server B.
> How do they typically manage things like scale, data growth, security, monetisation etc.?
Each to their own. Operators work independently. The operator of instance A is entire responsible for their own setup regardless of what happens at instance B. Operators get to set the rules on their own instance as to moderation, monetization, topics, community growth, etc. etc.
Since servers are interconnected, each instance can use an allowlist to determine which other instances, and their users, are allowed to connect and participate. As an operator, you get to decide that users of B can connect with your server, but users of C aren't allowed.
> Is the experience likely to vary a bit across the operators?
Communities are groups of people. Just like in real life, a community establishes its own culture, its own identity, its own way of communicating. So, yes, the experience can vary across the board depending on who's active on your instance.
Functionally speaking, all Lemmy instances operate the same way. You have communities, comment threads, voting, posting, etc. Barring configuration differences, as a user, the experience is quite similar across servers.
> And how do users trust which operators then want to join?
The same way you'd trust your ISP, your bank, the garage that services your car, etc. You read reviews, you try to gauge sentiment, number of users, etc. etc.
Mastodon - a Twitter clone - uses the exact same federation protocols as Lemmy. Mastodon has an order of magnitude more users across its instances. A big feature in Mastodon is the ability to just move your account from one instance to another instance. A thing Lemmy doesn't seem to have at the moment:
https://lemmy.eus/post/8181/comment/31995
https://docs.joinmastodon.org/user/moving/