Deleting one OS from Dual Boot

A couple of years ago I split my hard drive in order to hold onto Mojave to use an old version of ProTools whilst downloading Big Sur onto the Beta in order to buy Final Cut Pro. I'm now planning to abandon Mojave and carry on with Big Sur (or newer) and was wondering how I should go about crossing over. Should I delete the Mojave side of the hard drive or delete the Beta and update the original Mojave side?

Does the original OS take any priority on the drive?