Preview second screen in main one

Multi-monitor management software is very helpful for situations it sound like you are in, however to accomplish the specific goal of accessing windows on other desktops you mention it is easily done with hotkeys in Windows.

Use Alt-Tab until the window you'd like to move is selected, then press Windows-Left_arrow or Windows-Right_arrow to move the window between screens (these hotkeys are used alongside Windows-Up_arrow(maximize) and Windows-Down_arrow (minimize) for window management out of the box).

For previewing the contents of other screens, using something like Open Broadcaster is an option that not only gives you a preview of specific screens, windows, and select screen regions all on a customizable layout; it also allows you to record and even live-stream the contents of those regions with a lot of different control. The best part is that this software is free.