How did you choose the provider of your email address?
For Mastodon, you can check https://joinmastodon.org/servers to figure out what server you want to join. But joining one server doesn't mean you can't access people and communities from other servers.
It's one big network, but with many different access points (the mastodon servers), each access point has their own niche. These access points each can have their own rules, so be sure to read them before joining.
What the point of joining the fediverse is, I can't answer that for you, that's different for everyone. The only thing I can tell you is that there's a wealth of information available there.
“It’s one big network” People say that as if its meaning is self evident. I don’t know what that means. Does it mean that I can do one search and find anything on any server? If it’s one big network, why does it matter what server I choose?
Can you do one search on one server, and find any email address that exists anywhere? (not that I know of)
Can you send email regardless of what server the email lives on? (with some exceptions, but mostly yes)
Federation is not infinite or unlimited, so no, there is no "one search to rule them all" within the confines of a single instance. (But then there's no way to access all email, or search all email addresses across mail servers to find someone.) Still if you're on an instance, and go out and find people on the internet that interest you, regardless of how you find them or what instance you are on, you can follow them, and their content will begin to be federated into your feed, on the instance you use.
The server you choose matters, but it's also not something you can't change. (Like getting a new email address.) If you use Gmail, but someone uses a mail server that has been blacklisted because of bad players using a domain name or IP address, you probably can't get email sent from those servers.
But initially, you can either go with the biggest instance, or the first one you see on https://joinmastodon.org/servers that seems fine and then try it out for a while. It's not a single decision you have to make for the rest of your life. It's just something new, and a new way to have access to some people connected to each other through some instances, topics, follows, etc.
I don’t want to run my own server. But perhaps there is a server I want to join. There seems to be no clear way to search for them. You are referring to Mastodon, but the Fediverse is not just Mastodon.
Certainly. I have dipped my toes in a few other Fediverse pools.
Of course, like email / domain names, there's no universal search for picking them. Big ones get network affects or advertise. Small players have to be found organically.
That's a big flippin' list of Fediverse networks. But "a clear way to search" being layered on top of "decentralized" and "anyone could theoretically bring up a server at any time"... there's always going to be friction.
Whether you join any of them... well that's up to you to decide if you're curious enough, you suspect there's value there, etc.
The monetization and network affects of corporate social networks has always lent itself to incentivizing popularity contests and "emotional" content that pulls people in. If you want to escape the bad sides of those, there's also compromises on the independently funded networks.
For Mastodon, you can check https://joinmastodon.org/servers to figure out what server you want to join. But joining one server doesn't mean you can't access people and communities from other servers. It's one big network, but with many different access points (the mastodon servers), each access point has their own niche. These access points each can have their own rules, so be sure to read them before joining.
What the point of joining the fediverse is, I can't answer that for you, that's different for everyone. The only thing I can tell you is that there's a wealth of information available there.