I don't see how android file system is not getting fragmented over time. I suppose it does unless it is read only, and it's not. Also, FAT file system on memory card (if u use any) gets fragmented over time for sure. So actually there IS some point in defragmenting memory card. You can do it on your PC. Not sure about the internal storage. Perhaps it gets fragmented, but I doubt there's any tools which can defrag it
Some file managers put removed files in some kind of recycle bin. For example, AFAIR, FX file explorer does. But perhaps files removed by some applications via some internal API are not being moved anywhere, they just get deleted completely and the only way to get them back is to use some recovery tool like Recuva. However there's no guarantee that they don't get rewritten before you turn off your phone and try to restore your files.... Well, you see, it's complicated