The ROM is kinda an inaccurate term for the whole "blob" of binaries that gets copied to eMMC (or similar) storage. This can include multiple partitions, firmware updates (including for your baseband) etc.

The device kernel is a fork of https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin... or more accurately https://source.android.com/devices/architecture/kernel/andro... at the end of the day. The "whole thing as one commit" is just people not caring enough to maintain source history.

Treble seems to mean that the software can be updated separately from the drivers and the firmware - https://www.androidauthority.com/project-treble-818225/, it could actually make things worse in terms of out of date drivers and firmware.