Swiping applications off the screen doesn't actually close them. It just removes them from the list.
I think this is not actually true. The OS can close applications behind your back, but swiping an application off does remove it from memory, quite reliably. Things are done this way because Android devices have comparatively limited RAM and no swap space, so the system must proactively avoid memory pressure. This state of things might improve somewhat as recent Linux versions have added things like swap-to-compressed-memory and volatile memory that help save on RAM usage.
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