Mastodon has a big honkin' "Join mastodon.social" button both on joinmastodon.org and in the mobile apps to solve this exact problem.
Good solution. Problem is they have chosen a 'default instance' in the app which goes against the idea to encourage federation of the platform and instead accelerates centralization and there was backlash over that change. [0]
mastodon.social will only get more centralized and is the only one that is benefitting from that change.