It was GUI defined manually by pixel coordinates, having more flexible guis that could autoscale and other snazy things made things really "slow" back then..
Sure we could go back... Maybe we should. But there are lots of stuff we take for granted to day that were not available back then.
RISC OS has the concept of "OS units" which don't map directly onto pixels 1:1, and it was possible to fiddle with the ratio on the RiscPC from 1994 onwards, giving reasonably-scaled windows and icons in high-resolution modes such as 1080p.
That's only RISC OS 2 though. RISC OS 3 was 2MB, and even 3.7 didn't have everything in ROM as Acorn had introduced the !Boot directory for softloading a large amount of 'stuff' at boot time.
Sure we could go back... Maybe we should. But there are lots of stuff we take for granted to day that were not available back then.
It's hinted at in this tutorial, but you'd have to go through the Programmer's Reference Manual for the full details: https://www.stevefryatt.org.uk/risc-os/wimp-prog/window-theo...
RISC OS 3.5 (1994) was still 2MB in size, supplied on ROM.