Changing Mac to Windows then Back to Mac

You can use Boot Camp Assistant, Boot Camp - Official Apple Support

or you can use a virtual machine to install Windows on your Mac. If you use Boot Camp Assistant you can boot into either Windows or Mac. If you use a virtual machine both Windows and MacOS are running concurrently.

You don't "change it to Windows" you install Windows using Boot Camp Assistant or using a virtual machine like Parallels, Fusion, or VirtualBox.

What to do before you sell, give away, or trade in your Mac - Apple Support

Yes. If you use Boot Camp to install Windows you boot into whichever operating system, MacOS or Windows, you want to use at the moment. If you use a virtual machine Windows and MacOS are both running at the same time and you can use either.

with bootcamp one can set what operating system the next boot will be into

and if one hold down the option key at startup one can choose what operating system one wish to start

If you wipe your computer it won't have any operating system.

Explain what it is you are trying to do. Do you have a problem you are trying to fix? Is something not working for you?