GitHub "deletes" branches on merge, but it doesn't truly delete them - rather, it removes the branch, that's deleted, but the commits comprising the "pull request" are saved and referenced from that pull, so they never go away. They're slightly harder to find in GitHub and may disappear from your local copy, though.

There's a lot of other moving parts to trunk-based, they can only be taught by experience. I've written the tutorial where I have people make mistakes, and then fix them, but the problem is that they make mistakes in making mistakes.