How to use all the space in extracted hard disk (remove leftover system folders)
If you go into disk utility select view then show all devices. For the drive you want to erase, select the upper most text/name and then click erase. You can now chose the name, its file system (for normal use I would recommend MacOs Extended) and its scheme (you most likely want to have GUID). Unless its an SSD you can also select security options to securely erase the data.
Make sure that all files on all volumes and its partitions of that physical drive are backed up, since you will reformat and therefore erase the actual hard drive, not just a partition.

More info about the file systems: Apple support page, APFS vs MacOS Extended