The green button puts a window into full screen: it gets its own desktop, the menu bar and Dock hide, and switching to another app means swiping or pressing ⌘Tab. That is what confuses Windows users, because on Windows the same spot maximizes.
To maximize the Windows way, filling the screen with the menu bar and Dock still there:
- Hold ⌥ and click the green button.
- Double-click the title bar (System Settings → Desktop & Dock → "Double-click a window's title bar to" → Fill).
- Hover the green button and pick Fill from the menu that appears (macOS 15 and later), or press fn⌃F.
Full screen has its uses: a browser or a video with nothing else around. Leave it with the green button again or by moving the pointer to the top and pressing Esc.