> I don't want to learn a different UI for contributing to FOSS than the one I use for my daily work.
Codeberg is an instance of the FOSS forge platform Forgejo, which is a fork of Gitea. As FOSS projects, Forgejo and Gitea can be used to host any kind of software, proprietary or open source, without restriction. It is entirely possible for a person to use Forgejo/Gitea to contribute to both work projects and off-work projects, even if the work projects are proprietary.
If your employer chooses to use GitHub instead of Forgejo/Gitea, nobody else is obligated to stick with GitHub to make it more convenient for you to contribute. FOSS projects may choose Codeberg for its other advantages, such as its superior privacy policy and the fact that Codeberg does not automatically resell content from its hosted repositories as a proprietary service (like GitHub does with Copilot).