I grew up on RISC OS, and will forever fondly remember the desktop (especially 3.7 pre Win95), but when I've tried using it lately it is just a reminder of the folly of esoteric filesystems, on top of everything else.
It's kind of amazing the tenacity of people to manage to keep updating all this. Honestly I have no idea how they put up with things like the state of the build environments and so on.
It uses Acorn's own ADFS filesystem from the BBC Micro, which was released in 1981 if I remember correctly. MS-DOS and the IBM PC were new that year and not any kind of standard yet. The Acorn MOS and PC DOS are contemporaries. It would be a decade before Linux 0.01 existed, 14 years before FAT32.
It may look esoteric to you now, but it was a solid pragmatic choice in 1987 when this OS shipped, the same year as OS/2 1.0 and thus rather before HPFS which trailed OS/2 itself by several years as I recall.
Nice.