Here is the rough timetable:
Start download from Mac app store. This takes 20 minutes to an hour.
Once complete, start Installer. You specify disk on which to Install.
Install checks things out and then copies MacOS Base system to the destination drive. Time 20 minutes to as much as 3/4 hour if your drive is slow. (if installing on the same drive as the download, this step becomes trivial, and does not take any time).
Installer sets startup to the new MacOS Base System and does a Restart.
>> a failure right here means you still have everything on your old drive, (plus about 6GB of MacOS Base system that does not work)
ONLY if Restart was successful do you begin replacing files to move to the new version of macOS. Time 30 minutes to 3/4 hour.
Restart again and comes up running the new version, in Setup Assistant.