> Is raw a defined format?

It's not /dev/urandom written to disk, no. Yes, a raw format has a structure. There's not one "RAW" format though (and TFA notes this): e.g., my Canon's RAW format specifically referred to is called "CR3". And its predecessor was "CR2", so even within a manufacturer there are multiple such formats. All undocumented.

But a Pentax won't write out CR3s, it'll write out some other, yet bespoke format.

I've vaguely reverse engineered some of CR3: it is a container that contains multiple copies of the photo taken; IIRC it contains a thumbnail JPEG, the JPEG, and the raw data itself.

I doubt it's the most performant to write to storage: the format is vaguely TLV (its fairly similar to RIFF, if you understand the RIFF format), so it can't really be streamed to storage due to needing to know the lengths of the containing chunks (all the way out to the outermost chunk).

By defined format I of course don't mean random, or that it doesn't have structure, you have to be able to read it back otherwise what would be the point. I mean when you write out jpeg it has to adhere to a standard otherwise it isnt jpeg. Granted the jpeg format has evolved and has been augmented over the years. But it still is standard across iterations and devices.