Key folders in the Trash
After my iMac (late 2012) was repaired and upgraded to High Sierra, four empty folders appeared in the Trash, which I can’t empty because they’re “in use.” They’re named Applications, System, Library, and Backups.Backupdb. How can I get rid of them? Your iMac’s upgrade to High Sierra did not complete correctly. The first three of those are key top-level folders from your startup disk, which respectively contain all your apps, macOS system files, and many important support files for all users. The fourth is the top-level folder from your Time Machine backup drive.
However those folders got into your Trash, they should not be there at the end of a normal High Sierra installation. Since they’re reported as being “in use,” they could be the result of corruption to your startup volume.
If your repairer is a competent Mac techie, they should be able to sort this out. That would normally involve restarting in macOS Recovery, opening Disk Utility from there, and using the First Aid tool to ensure that the disk is healthy. They’ll then need to ascertain the origin of those folders, and check that the folders of the same names currently on your disk are the correct ones. Ultimately, they might have to format the disk and reinstall from scratch, or those could lead to long-term problems.