The desktop application supports you giving the app your credentials to BitBucket, GitHub, and GitLab, whereby it then uses your credentials (it uses your username and password instead of an API key or SSH key??????). See the docs here:

https://developer.apple.com/documentation/xcode/configuring-...

> This step is necessary because Xcode Cloud uses your personal SCM account to monitor the Git repository for changes.

From that point on, Xcode Cloud polls/monitors the repo for changes, but it does so in a way that doesn't integrate with CI/CD.