Anyone can open a PR. Deploys are triggered by an Immich collaborator labelling the PR, but it doesn't require them to review or approve the code being deployed.

As I've mentioned in several other comments in this thread by now: The whole preview functionality only works for internal PRs, untrusted ones would never even make it to deployment.

Yes, but unless that pr contain malicious code domain shouldn't be marked as such. You should assume good faith, not the other way around.