I can’t speak for any specific instance, but I believe many of the smaller ones that defederated lemmy.world were concerned that a large user base with looser sign-up requirements would disrupt the community they wanted to build.

This is mostly correct, but another point to consider (once OP becomes more familiar with the basics) is the concept of defederation. Basically, not all instances permit interactions with each other. Many smaller instances have defederated from lemmy.world, for example. That means that users on those instances can not directly interact with communities hosted on lemmy.world, although you could still encounter users from that instance in a post on a community in a different instance that is federated with both.

If the instance you are currently on has defederated from another one that you would like to browse, you will need to create a new user account on that instance (or one that hasn’t defederated from it).

I have a government-issued iPhone. It has Signal on it, and I have access to the appstore. It’s also not allowed to be used for anything but unclassified communication and isn’t permitted inside restricted areas.