How do you quit Garageband on a Mac?

With GarageBand in front and on the menu bar, use Command + Q or select GarageBand then Quit.

Another way is to Control (right) click on the GarageBand Icon on your Dock and select Quit.

see > Quit apps on Mac - Apple Support

+1 to den.thed. I almost exclusively use Command + Q to close apps. Simple and much quicker than using a mouse. You may want to spend a little time learning some common keyboard commands such as cutting and pasting (Command + C & Command + V), you can find a good starting library of them by using the info in Mac keyboard shortcuts - Apple Support