Fast User Switching has been a mainstay feature the macOS for many years. People use it to have several family user or business accounts on one machine. Tech guys use it to have different user accounts that are set up in certain ways for troubleshooting. macOS Big Sur comes with Fast User Switching turned off. Here is how you activate it, if you are inclined to use it.
Fast User Switching
The Fast User Switching feature is located in the Dock & Menu Bar Preference Pane.
Once in the Dock & Menu Bar area, scroll down and select Fast User Switching in the left sidebar.
Now, all you have to do is check the box for either “Show in Control Center” or “Show in Menu Bar”.
I chose the Control Center. Here is what you see in the Control Center drop down.
You just click on the person icon to get the Fast User Switching menu.
When you select another account, your Mac is logged into that account.
Fast User Switching is the same in Big Sur as previous macOS versions. It is just accessed a little differently.