Applications keep opening on monitor I dont want them to open to

Keep in mind, the green button on the upper left corner of the window has two modes. By default, when it has the opposing triangles, it maximizes the window and covers the entire screen:

If you hold the Option/Alt key before clicking on it, it will turn into a plus, and maximize on the screen within the bounds of the menubar are Dock of that screen in question that it covers the most area of. Once maximized in the window, it usually remembers that position when you quit the program and/or reboot the computer with the applications remembered at their last positions. If you drag the center of a window near its titlebar without holding any keys you can move that window onto another screen before doing the maximize. If you option key after maximizing, it will shrink down to the last/size position it was at. Note, this is not really practical when you have multiple tabs open on a web browser because then it tries to move around the tabs instead of the whole window.