What parts of furniture do you varnish e.g. inside drawers?

This must be a silly question because I can't find the answer after lots of googling, but here goes... When staining and vanishing wooden furniture, which parts do people varnish? Obviously the visible outside bits, but what about the back or underneath? Or inside drawers and cupboards? I know the answer is on some level "whatever I prefer" but I'm guessing there's a consensus on what looks most realistic? (I don't own any real wooden furniture to check the real thing!) If you're vanishing inside, how're you dealing with sanding between coats? Also I'm going to fill the grain before staining, but the first/only time I tried that before, the flat top of the table turned out beautiful but the details (e.g. legs) looked scrappy in comparison (I don't think I filled them because I knew I wouldn't be able to sand it back properly). Is there a trick to getting the details looking lovely too? Thanks in advance for any advice 😊