What do you mean with locked down and secret, all the code is freely available and no secret and how locked down your system is dependa on how you install it. The more locked down the system (Home Assistant OS is the most limited), the more convenient and user friendly it gets. If you want to have full control over everything you need to install Home Assistant Core in a virtualenv, but then you don’t have any add-ons, supervisor etc. and you need to know your linux and python.
Here’s an excellent overview of the different install methods.