While some ideas like hierarchical filesystems were new it was mainly a modernized version of CTSS according to Dennis Ritchie's paper "The UNIX Time-sharing SystemA Retrospective"

I was playing with this version on simh way too late last night, taking a break from ITS, and being very familiar with v7 2.11 etc.. It is quite clearly very cut down.

I think being written in Assembly, which they produced by copying the DEC PAL-11R helped a lot.

If you look through the v1 here:

https://www.tuhs.org/Archive/Distributions/Research/Dennis_v...

It is already very modular, and obviously helped by dmr's MIT work:

https://people.csail.mit.edu/meyer/meyer-ritchie.pdf

But yet...work for years making an ultra complex OS that intended to provide 'utility scale' compute, and writing a fairly simple OS for a tiny mini would be much easier....if not so for us mortals.

It isn't like they just came out of a code boot camp...they needed the tacit knowledge and experience to push out 100K+ lines in one year from two people over 300bps terminals etc...