Hello @widkit, this issue sounds like the same Flatpak issue as https://github.com/flathub/com.valvesoftware.Steam/wiki#i-want-to-add-external-disk-for-steam-libraries / https://github.com/flathub/com.valvesoftware.Steam/issues/55. Your issue is with the nature of Flatpak's sandboxing and not Steam.
Nothing extracted yet.
I can't add my Steam Library folder, 'cause it doesn't exist? I guess?
The folder exists and mounted on the correct directory.
here, the folder "Downloaded Program Files" has SteamLibrary folder inside.
but there is no plus sign like any other folder with subfolders.
Steps for reproducing this issue: