This is fixing issues in the packaging, not the upstream part of the package. I have plans to also have it fix issues in upstreams, but as you say - those will go into the upstream repositories.

It processes the main Debian archive, not PPAs. The bot makes changes to the Git repository with the packaging - developers would get a pull request with the changes after doing verification, and they can just click "Merge" on the GitLab UI. That seems pretty similar to what you're saying would happen in arch.

For a broader description of the design, see https://jelmer.uk/debian-janitor.html