For a visual you can essentially type anything that you file path contains to populate a playlist will all files that contain similar “paths” or “filenames”
If you wanted to use a folder for this you just need to type the name of the folder in (without quotes)
In this case my path includes a folder & sub folder
Music &
Downloads (a folder within music)
So i could type either of those values although the results would differ greatly.
My filenames often have “codes” at the end. This one has (NUKE) so i could also create a playlist to grab all files with that “code” in their filename.
Similarly many people add artist names, album names, track numbers ect. to track filenames so those are also bits of info you could use to filter your library or create smart playlists
If you dont know the file path you can find that by browsing your file manager or using the “3dots” more actions menu on any given track to pull up this view which shows the file path at the top