Raspian originally was a custom build of Debian to target the unusual hardware support of the 1st gen Pi (it had hardware float but not other bits needed for the armhf architecture, e.g. like having SSE but no PAE on x86, I can't remember the details though). Newer Pis are more standard (see "Are the images 32 or 64 bits?" at https://raspi.debian.net/faq/), and so Raspian isn't needed for them. So that's the difference between Debian and Raspian (there's likely other differences with extra things the RPi foundation has added, but YMMV if you find them useful or not).

For SoCs, you can either do the installation on the device (which can be painful), or build a chroot with the needed features and flash it to hardware (e.g. sdcard or some board storage), https://raspi.debian.net/ is providing pre-built chroots for you to flash (but if you wanted, you could use https://salsa.debian.org/raspi-team/image-specs to build/customise your own). There's lots of similar projects which build a Debian chroot for a specific SoC, either with custom kernels or with a specific feature-set (e.g. freedombox: https://www.freedomboxfoundation.org/). None of these are "official" Debian projects (I think debian installer a.k.a. d-i is the only official/supported way of doing a install), but many are run by Debian Developers scratching a specific itch.

FYI, if you look at https://salsa.debian.org/raspi-team/image-specs/-/commits/ma..., there appears to be a fair few people who've made changes, and a least a few of them I recognise as being DDs, so it's not just one person (Gunnar Wolf is probably the person the name's been allocated to, and he probably wrote the docs). If it's security support you're thinking about, I'd put those images as more likely to have a fix early than Raspian (as those images are stock Debian, not rebuilds).