I think you'll benefit from reading this page:
https://musicbee.fandom.com/wiki/PlaylistsYou can manually add and remove files from an auto playlist (see the link above for details), but if you have a playlist based on genre the best thing to do would be to change the genre tags on your files to include or remove the value, and then the tracks will be added to our removed from the playlist accordingly. An auto playlist is, as the name suggests, automatically generated.
For more than one genre use "is any of" and list the values, separated by semicolons. For instance "Rock; Jazz; Blues".
You can add by drag-and-drop to either static or auto playlists, although as I said above, in the case of an auto playlist it's probably better to just change the tags so they match the playlist rules. I suspect you're thinking of static playlists, which are entirely manual. Another thing I strongly suggest you spend some time with the wiki for.