- Find an extension that adds things in the same place you need and check which component they used.
- Read Flarum source code to find the correct place.
Reading the source code becomes easier the more you do it. For your particular screenshot, I'd know it's the
IndexPage component that renders that page, so I'd go to that file, then under the
view method I see most of the page layout, with
sidebarItems on the side, and
actionItems just above the
DiscussionList component. It's then easy to confirm
viewItems are the controls on the top left and
actionItems are the controls on the top right. I'm not sure if that's what you were actually looking for though.
In the past I created this extension that would visually show you all the extendable points of the Flarum frontend but it was never fully finished https://github.com/migratetoflarum/flarum-ext-learn
There's also ways to insert things in other places of the frontend even if there are no ItemLists to extend, but it's a bit more difficult and requires knowledge about how Mithril vdom works.