Mac storage basics · Memory, swap and speed
Does quitting apps free up disk space?
Short answer: Mostly it frees memory. Swap and some temporary files shrink after apps quit or a restart, but caches, downloads and app data stay until you remove them.
Mostly it frees memory, not disk space. A few things on disk do shrink after apps quit or the Mac restarts: swap files used under memory pressure, and temporary files some apps delete when they close.
That's why free space sometimes jumps after a restart. But caches, downloads and app data stay until you remove them, so quitting apps isn't a way to clean up storage. See why System Data shrinks after a restart.