> so the diff in a PR shows just the relevant changes from the specific branch

That's exactly right.

> you can create stacked PRs purely via the UI

How?

I see from the docs https://github.github.com/gh-stack/introduction/overview:

> When a pull request is part of a stack

How does GitHub determine if a PR is part of a stack? Is it automatically detected so that I don't need to adjust my tooling that already creates chained PRs?