For me, that would be IT for the company using the product (Tridion Sites or Docs in this case).

Knowing what I know now, at least for Sites, the "Administrators" should be members of the Admin group(s). If the "task" workflow was more popular, I'd even send a task to the admin with the message details.

But nowadays, I'm assuming email or chat integration would be a better way to "contact" an admin.

Some companies have 3rd parties or other externals administering a given system. In that case, admin should be clear based on the given system (whoever you normally ask for system help from for that system).