that's due to the way information is populated, while your instance can see the content, it doesn't actually know its there until you search for it which populates that data (in simple terms anyway)
it basically comes down to personal preference tbh, each instance have slightly different rule sets and internal communities, lemmy.ml / lemmy.world are simply some of the 'main' instances, They're higher on the list on the instance search and thus generally gather a larger crowd because people see the higher user count and just go with that.
and if you don't like any of the public instances you can simply roll your own and use it as an internal friends chat that can also browse the external fediverse
No, you have to make them per instance, but its sorta redundant as each instance communicates with each other and displays the same content / can communicate with each other (example: You're lemmy.sdf, The user below me is Beehaw, and I'm shi.tjust.works
Don't think I'll go as far as deleting my account as it still holds some sentimental value to it (things nearly 12 years old at this point) but as for browsing / general usage, yes, I've fully switched to Lemmy as my daily site. While there are a few niche subreddits I'll probably still periodically check in on the sites been going downhill for years now, and the most recent AMA / API tomfuckery made me jump ship to greener pastures.
For all of Lemmy: https://lemmy.nz/communities/listing_type/All/page/1
For just your instance: https://lemmy.nz/communities/listing_type/Local/page/1
There isn't really a way to search by topic as of right now, but you can search in those menu's with generalized subjects to find related communities
Instances are effectively just different versions of the site, they all communicate with each other and display the same data (roughly), like someone said in a different post, think of it like Email, wherever you made your account being your host domain (An example being look at mine and your username, you are posting from Beehaw, while I'm from Sh.itjust.works, different instances, same content.)
