Edit, thanks to comments below from @petefoth and @smu44, will also hide stuff from everyday apps like Music and Gallery so is not suitable on “everyday folders”.
Any folder containing an empty .nomedia file will not be synced.
This file can almost always be found / generated with the Files app. Use the search facility of Files to search for .nomedia in the Internal storage of the device.
Select one > 3 dot menu top right > Copy to … and select the folder you want to exclude from sync.
You might create a My_Stuff folder, put a .nomedia file there, then move files in.
I don’t think I have tested what happens if you put a .nomedia file into an already populated folder for instance Music. Edit, it is a bad idea as your Music may well be hidden from your Music player.