> If you don't explicitly minimize the window and open another program, the other window is hidden. Where is it? How can I open it? Yes by minimizing every window until I have found mine.
I don't understand exactly what you mean.
> what happens if you have several windows open of that app?
One of these:
• Right-click/Control-click on the Dock icon
• Check the Windows menu of the app. Minimized windows will have a Diamond
• Press F3 to open Mission Control.
• Press Control+F3 to see all windows of the currently focused app.
• Press Alt+F3 to open Mission Control settings and configure them to your liking, along with setting Hot Corners for showing application windows etc.
• If you "Group windows by application" and have a mouse with a scroll wheel, you can use scroll the wheel when hovering over an app's window, to "spread" that windows stack.
• Press Option+Command+H to hide (not minimize) all windows except the active app.