This article
references this one
which in turn references these two articles:
and this really excellent one
(Note that all four aforementioned articles are translated to English by Google when you click on them.)
The last one provides a very good and concise history of Sailfish OS, Aurora OS and the collaboration between Jolla and Rostelecom / its subsidiary OMP!
Now I wish that some customer of Aurora OS 5 executes his right provided by most FLOSS-licenses to demand the source code and publishes it.
Mind that this is very unlikely, because Aurora OS primarily targets governmental institutions and secondarily corporations, not private users.