Funnily enough, in a world with WASM, we might actually have Java in the backend and C in the frontend rather than vice versa as it would've been likelier in the 90s.
The smallest transfer done from memory is a single cache line, which on most desktop machines is 64 bytes, or 512 bits. You could imagine a memory bus that was 512 bits wide and transferred a cache line per clock, and this would improve latency when compared to a serial bus with higher clock speed. HBM doesn't do that, though, instead every HBM3 module has 16 individual 64-bit channels, with 8n prefetch (that is, when you send a single request to a single channel, it will respond with 512 bits over 8 cycles).
Not likely, unless they make a headset which doesn't do much of anything by itself and is just meant for streaming from a PC. To make a standalone headset which can draw on the Steam catalog they would almost certainly want to use a variant of their SteamOS Linux distro.
Valve already has its own OS for a mobile device: Steam OS (based on Arch) on the Steam Deck. My bet is that they'd just modify that for a standalone headset.
I used to query databases and create diagrams from the metadata, but text to diagram just makes little sense to me.