Sanity check:
According to
https://en.wikipedia.org/wiki/IPv6_address the most significant bits are those of the network address prefix aka routing prefix. That makes sense as msb-on-the-left is how we generally write numbers in hex or decimal or whatever base.
In the IPv6 header, within the addresses, which bytes ie at what offset are the most significant bits of the addresses? Is it the usual big-endian format that is popular in IP? That is msb first, at the lowest address.