How do I compare my branch with main?

Select the tip commit of one branch in the history, command-click the tip of the other, then right-click either of them. The first item in the menu is the comparison, named as the two short hashes with an arrow between them. The result opens in the Diff Review window as an ordinary diff.

Order is not something you can get wrong. Innesta puts the older commit on the left whichever one you picked first, so the diff always reads as what would have to happen to the earlier one to become the later.

Just the commits

Clicking a branch in the sidebar scopes the history to it without checking it out, so flicking between the two is two clicks. The graph beside the list shows where they diverged.