I think the assumption is that if you're interested in the source or contributing, you already know what it looks like.

There are loads of galleries and walkthroughs out there.

Here's v3.11 from 1992:

https://guidebookgallery.org/screenshots/riscos311

Here's the more modern 3.7 from 1996:

https://guidebookgallery.org/screenshots/riscos37

An annotated walkthrough the GUI:

https://telcontar.net/Misc/GUI/RISCOS/

there are plenty of screenshots in the user guide:

https://www.riscosopen.org/zipfiles/platform/common/UserGuid...