Neither Windows nor Android have a (generally-used) concept of quitting an entire application at the user interface level. You can force-quit a process, but that's a rarely-used feature intended primarily for debugging purposes (and accessible through a side-channel; Process Explorer for Windows, app settings for Android). Some applications implement such a concept, but they're generally "single-window" apps, where closing their sole view is equivalent to closing the entire app (games usually fit this category).

Nowadays, you can generally trust your OS to page an app in and out and devote no resources to it if the resources aren't needed. Quitting an app is for misbehaving apps, which isn't assumed to be the common case.