My wife does not use her iMac and has offered me hers which is newer and more powerful. If I go to her device and create an account using my current iMac information, what happens to all data on both devices?
[Re-Titled by Moderator]
iMac 21.5″, macOS 10.15
If you create a new user account on her computer it will be completely separate from hers. If you have iCloud set to sync data (multiple options), the data will eventually transfer to her computer. However, you may wish to look at a more complete option which will get non-iCloud things too. Transfer to a new Mac with Migration Assistant - Apple Support
If she really does not want her Mac at all I suggest you first reset it completely so it is "yours."
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
If you create a new user account on her computer it will be completely separate from hers. If you have iCloud set to sync data (multiple options), the data will eventually transfer to her computer. However, you may wish to look at a more complete option which will get non-iCloud things too. Transfer to a new Mac with Migration Assistant - Apple Support
If she really does not want her Mac at all I suggest you first reset it completely so it is "yours."
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support
What happens to data when creating a new user account on iMac?
