Moving on from folder browsing, there are some things that can be done to enable easy access to folders that contain music that may not fit usual Focus criteria for easy bookmarking.
Firstly, the Focus/Inspector/Storage locations tool enables you to Focus on top level folders, so if you have a folder of recent music on your desktop copied from a USB stick, for example, you can add that folder as a top level Roon folder and Focus on it with this tool.
Roon Tags are a way of organising music outside of Focus tools or folder structure. You can create a Tag by selecting all albums within a view and add to it from other views or Focus bookmarks.
There is a longstanding feature request for Roon to recognise a file tag created in a third party tagging program and use that file tag to generate or add to a Roon tag. Roon has already implemented ROONRADIOBAN file tags so there is good reason to be optimistic about this request.
If you get stuck with a particular use case then there are many experienced users on this Forum who are willing to describe what they do or what can be done.