I'm still not sure I understand how communities and instances work.
If I create an instance here will it show in other instances? Because I did create one on Lemmy world and I can't see it in the other instance I am at with my other account, even though I look at "all"
When you create a new community, other instances don't automatically know it exists yet. They find out when the first person from there, searches for your community (search syntax is given in the community sidebar: !community@instance <-- note the ! at the start). Once someone subscribes, it shows up in that instance's All feed.
To get the word out to other instances you can post in the various relevant communities as well as make sure to drop the link into any relevant conversations where people might be interested in checking it out. Since you have at least two accounts, if you create a community on one of your instances you can also use your other account as a shortcut and search for it yourself on the other instance.
An instance is just a single distinct computer (server) running the Lemmy software. You have a bunch of these separate computers running the Lemmy software. These computers - together - form the Lemmyverse. (I'm purposely leaving out Fediverse, activitypub).
Each user (no matter what computer/instance they signed up with) has the ability to comment on any post made within this system of cooperating computers (The Lemmyverse). We can also subscribed to each other's communities (ie; forums, subreddits).
That's basically it. The ability to share posts and to comment on each other's posts. You can't use your login details across Lemmyverse since each computer is distinct.
Some of these distinct computers may decide they don't want to be part of this Federation of cooperating computers. For the most part they can disengage from this Lemmyverse. For the most part... but the software is still on version (about) 0.18.2 and so complete (or temporary) disengagement from Lemmyverse is still in development (ie; coding, logic decisions, etc).
Excellent!
It's like old days php forums but you have feeds from other forums too.
It's a bit scary though that you can have whole lot of content and a whole community but it's all up to the single instance host if they want to drop it from one day to another.