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.