It takes consensus of 17 of the top 20 witnesses to change the code or core parameters. The HBD savings interest rate is a vote amongst witnesses and doesn't require a hard fork.
Hive is completly open source. It was forked from Steem after Justin Sun bought Steemit.
https://gitlab.syncad.com/hive
I'm looking for a decent explanation of governance but ultimately the White Paper is the source of truth. https://hive.io/whitepaper.pdf
Basically top 20 Witnesses produce blocks every 3 seconds. A backup roster of around 100 more witness produce the 21st block in each rotation.
Witness ranking is by votes according to the staked Hive (Hive Power) held by the voter. I’m witness 50 right now and that has taken me lots of work to earn community respect.
Discussion
Thank you