In the plugins API:

• Yes if the plugin is private within the organization, you can get the file key. Plugin Manifest · Figma Developers & figma · Figma Developers
  


• If the plugin is public in the Community, the user would have to paste in the key.

In the REST API:

• You can get all files within the team, but the user still needs to paste in the team link.

Thank you for the super-fast answer!

I somehow missed that it’s already (while indirect) in the REST API docs 🙈 (for anyone who also misses this)