virtual box

If you look at the videos for installing Ubuntu on a mac,VB, it says to choose the install choice that says " erase disk and install Ubuntu. THis will erase all files of any OS etc".
Just to be sure ...its ok to select this because you are in VB and not the real hard drive environment..,yes??

Yes, everything is virtual.

good.........

Good sense dictates that you should have a backup of your macOS before proceeding with VirtualBox just in case something goes wrong.