Spaces, the Dock, multiple screens, very easy to get into a whirl with those things. I must say I don't feel I really achived 100% control of them after years of trying, but I do have 2 or 3 screens with separate spaces set up to a good degree of functionality.
Just a couple of tips; much is achieved by experimenting with the settings that show when you option-click on an active app in the Dock. "assign to... Desktop1, None... etc." lets you set a preferred screen/space for that app. If you don't see the options, it's because you have only one space (or "desktop"), if so create a new space, and the options will show.
Sometimes an app will seem to not follow the rules and show a new window on a screen that is not set in the options, but I think that's the developer's fault.