EOS-3.x varies depending which release you are asking about. EOS-3.9 is based on Debian Buster.
At this point I would suggest focusing your efforts on EOS-4 as it will be released soon and is much closer to Debian than before. It is based on Debian Bullseye. Most of the packages come from Debian Bullseye directly, but a small number of them are overridden and modified by Endless (apt repo is based at https://deb.endlessos.org/).
EOS-4 does include a major update to the printer/scanner support, so you might want to double check your device there if you haven’t already. We are now focusing our support on those printers and scanners that support driverless standards (i.e. printers and scanners that can be used with a smartphone).
If your device does still require a funky package direct from the manufacturer then that’s not something we support officially, but you’re welcome to open things up and try it on your end - in which case you’d want a deb package compatible with Debian Bullseye.
v4l2loopback is much harder since it is a kernel driver and unlike installing extra deb packages, that’s more unchartered territory. The best “moving forward” answer there would be to have it included in the official Linux kernel, then it will become available in Endless (& all other distros) without any installation process. I think that’s unlikely though, since v4l2loopback being on the kernel side doesn’t seem to be an optimal solution for solving that problem. Instead I would expect this functionality to be introduced in userspace via PipeWire at some point going forward. (Endless will adopt PipeWire along with Debian in due course)