How do you disable desktops?

Use Zoom instead of Full Screen.

There isn't a Maximize function in macOS, but there is Zoom which toggles between a user set size and the smallest size that will contain all of the content. Since the smallest size to contain the content is usually the entire screen, then Zoom usually sets the window to the full screen size.

You can get to Zoom in the Window menu or by holding down the Option key when clicking the green dot.

You can also set Dock System preferences to allow double-clicking in the title bar of a window to invoke Zoom.

Finally, Full Screen causes the other monitor to go black because you have your monitors set up to expand across both instead of having each display act independently. You can adjust that in Mission Control System preferences. I would use Zoom regardless. I think Full Screen is the stupidest Apple design idea ever.

Maximizing a window is doing exactly that. Maximizing it to the full size of your screen. Are you familiar with swiping between full-screen apps and Mission Control?

When you expand a window to full-screen, your desktop is untouched. Using two fingers on your MagicMouse, or three fingers on your trackpad, you swipe left or right to go see your desktop.

Or.. with two fingers on your Magic Mouse, double-tap your mouse to see your desktops, or swipe up on your trackpad to see the same thing.

For people not familiar with it, it takes a little bit of time to get used to it. However, once mastered it's a fantastic way to manage multiple desktops. I encourage you to use it more. It's one of the great features of MacOS that I've used for years.

Hope this helps.

That being said, to my knowledge there is no way to turn the maximize feature off.