How do I stop Windows from automatically maximizing?

To turn this feature off simply open up the Control Panel, then the Ease of Access Center. Click on the section called Make the mouse easier to use and check the box next to Prevent windows from being automatically arranged when moved to the edge of the screen.

How do I stop Windows from snapping full screen?

In the System Settings window, click the “Multitasking” tab over on the left. On the right, you’ll see several options in the “Snap” section. You can use those to turn off different aspects of the Snap feature, like whether dragging windows snaps them at all and whether snapped windows resize to fill available space.