currently, they don’t, but the mobile operating systems do heavily cache applications in ram and those applications are getting pretty heavy, so instead of accessing bulk storage every time you launch an app, they are pretty much all sleeping in a low-cpu state with most of their content pre-loaded into ram.
Makes them snappier to switch apps and such, particularly with image or video heavy apps that want to be able to preload pages and pages of content to mindlessly scroll through while the internet connection loads more in the background.
Windows and MacOS has started doing a bit more of this, but still gets by with less ram as you tend to have a more powerful CPU and faster storage to not worry about an extra fraction of a second to switch between apps, but 0.5 seconds to switch apps on a phone can feel slow because we humans are simple dumb creatures with zero patience.