This used to be easy - simply drag whatever files you want to onto a Time Machine disk. Now, macOS reserves the entire APFS disk for Time Machine backups and you are no longer able to drag files onto the disk. You need to create an additional APFS volume on the drive and use that for general file storage.

• Command + Space > Disk Utility > View (from top menu) > All devices > Click on the drive
• Then click on Partition > the + button > Add volume > then give it a name and choose APFS encrypted (if you want the volume encrypted)

Then go into Time Machine and set whichever volume you want for TM backups