To add more clarity here in case it's helpful: If you open the Rectangle app's settings menu and open the last tab (looks like a gear), you can play around with the "Gaps between windows" setting.

I have this set to 15px, so when I drag my app windows around, they snap to a grid with 15px spaces between the windows. Dunno why I find it so satisfying, but it really scratches an itch in my brain.