How do I erase all user accounts w/o erasing Mac startup material?

My spouse got a new Mac Studio (I know, timing is everything) to which she transferred all data/files/software from her iMac Pro backup drive; I'm inheriting that iMac Pro to which I want to transfer all data/files/software from my '13 Mac Pro backup drive. Two questions:

1) How do I erase her iMac Pro data/files/software w/o erasing the basic Mac startup process I'll need to use to transfer the data from my '13 Mac Pro backup drive? (i.e. how do I avoid erasing everything)

2) Will there be a problem in transferring from my Mac Pro backup drive in that it is running Monterey 12.6.6 instead of the Ventura system on the iMac Pro?

Thanks in advance.