Hi @Anonymous ,
I will try to explain how all the library stuff works...
First of all how to use EAGLE libraries in Fusion?
If you have a local EAGLE library and want to transfer it into your Fusion environment you can simply upload it.
In the Fusion data panel, navigate to the folder where you wold like to see your library stored and click the Upload button. Select your local EAGLE library and that's it. All libraries that are in your WIP (Fusion cloud workspace/storage) can be used immediately. In the Fusion Library manager this library will be marked as "local" type.

If you are working with Managed EAGLE libraries, your libraries are stored in library.io. In Fusion you have access to EAGLE libraries in library.io through the Fusion Library Manager. Open the Library manager in Fusion and go to the Available tab. There you will see all libraries that are available on library.io. Select those you want to use.

That' all about using libraries.
Now next step about editing libraries:
In Fusion you simply open a Fusion library that is stored on WIP you can add components, create 3D packages and footprints...... and save it afterwards. No matter, if you created it before from scratch in Fusion or uploaded an local EAGLE library from your computer.
If you would like to add content to one of the existing managed EAGLE libraries, you have to edit it and then save it in your Fusion WIP. How to edit? Go into the Fusion Library Manager and select your library from library.io from the list in the In use tab and click the Edit button. If your library is not listed there, go to the Available tab and Use it.

As soon as you save this library it is stored in your Fusion environment. The changes will not be transferred back to library.io automatically.
Which leads us to the next topic about synchronisation of libraries in EAGLE and Fusion.
Given you edited in Fusion a library that comes from EAGLE (aka Managed library stored at library.io) and you want to bring these changed back to library.io because fore example your colleagues still works with EAGLE, you can sync the library and Push changes to manages library on library.io. therefore click the arrow up icon in the library editor.

Of course, tis also works the other way. If somebody makes changes to an managed library in EAGLE and you want to have this changes available in your Fusion library that came originally from library.io, you use the arrow down icon next to the marked icon in the image above.
I hope this helps to understand how this works. Happy to hear from you.
Best regards,
Richard Hammerl