Wiping an old computer for future sale

I have a MacBook Air 2012 that I am wiping so that I can sell.  I want to make sure it's good and clean before I sell it. So I booted under Disk Utility and then I reset it to the factory settings. I had just updated it to macOS Catalina. Everything worked fine until I noticed that the data volume didn't change. There was still only 33GB free space and the rest was still there clogging up the computer. So I rebooted the computer again and deleted the data on it but not the whole volume as I was scared I was going to delete system files and wreck the computer. So, this is what happened. I still have 86.98 GB used and only 33.14 of free space, plus it added a disk image to the Disk Utility and under that, there is another folder that says macOS Base System. What is happening? What do I have to do to free up all the space and erase all my data so I can sell it? Please help me!