I just added repository A to my Git perspective in Eclipse (which also instanced a local Git repository under some path) and can now see the working tree. Now how do I work on the files in the package explorer?
From what I've seen I have to use "import projects" - what does this do? My Eclipse workspace seems to be empty, while made changes will all display in the git repository.
Also, how do I work with multiple repositories and how do I switch between them? When I try to "import projects" from repository B I get the message that Some or all projects cannot be imported because they already exist in the workspace
The issue here is that Eclipse maintains its own file system representation that includes projects. This was required historically to support incremental compilers, even if the file system is not capable of sending notifications.
The Import projects option you have seen relates to importing projects from a git repository into the Eclipse file system; adding the repository to Eclipse allows Eclipse to know about the repository, and execute repository operations (e.g. fetch, commit, push).
About why doesn't the project import work, the Eclipse file system contains a set of top-level projects (each can be either in the workspace folder or somewhere on the disk, e.g. in a git repository); however, it is not possible to have two projects with the same name in the same workspace.
If you want to open the "same" project from two repositories (e.g. two development branches), you have to rename them not to have a common name. This name is stored in a .project file in the project root (or if none exists, it will be derived); you can use a refactor operation to change the project names. After that, you can import both projects into your Eclipse workspace.