I've messed something up whilst trying to factory reset my Mac for a new user.

While in Internet Recovery Mode

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives.

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. 

6 - Once that is done >> backup out of Disk Utilities and choose install macOS.

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

When i erase MacHD, it doen't go away, it just stays there.

Erase in Disk Utility doesn't make it go away. It replaces the format on the volume with another one (or maybe the same), but it doesn't remove the file system altogether.

If you attempted to Erase the bare drive in Recovery, it would not be able to because the Recovery OS is running from that drive.

You should be able to erase (i.e. reformat) the MacHD volume. However, I seem to remember it always names it Untitled, so if you didn't change the name in the Erase dialog, it may not be actually erasing that volume.

If you boot into Internet Recovery (cmd-opt-r) you should be able to erase the bare drive. Use GUID and APFS.

Catalina should be able to install on HFS+, but it will convert it to APFS.

If you are trying to install Big Sur (which recovery might be trying to do), it would need to be formatted APFS.

Can you erase MacHD?

When you erased, did you use GUID Partition Map and APFS format?