You can hide the menu bar in macOS Big Sur. Personally, I wouidn’t recommend it, bu here’s how:
Open System Preferences.
Click on the Dock & Menu Bar icon.
About halfway down the new window, you’ll see an option to “Automatically hide and show the menu bar.” Click this box and the menu bar vanishes. It will show up again when you glide your mouse arrow towards the top of the screen.
Don’t like it? To make the menu bar stay visible at all times, recheck the “Automatically hide and show the menu bar.”