I've worked in industrial process systems where firmware is pushed out to millions of devices. The process is pretty rigorous - first the engineering team develops a candidate build, they run it through full regression and acceptance. Once it passes all the various code quality tests (coverage, static analysis, etc...), it's signed. Only code which is signed can be run on the production bootloader. The process required to sign a firmware image requires M of N people to come together with their credentials (hardware keys) to have the HSM (Hardware Security Module), sign the firmware.

The actual roll out of the firmware is of course staged - in Tesla's case, they likely roll it out to all of their employees teslas for a period of time, and then to a candidate group, and eventually everyone.

The team that actually pushes the firmware, is completely separate from the team that builds, and signs - they have a fairly detailed set of procedures that ensures a quality push, which is their entire focus.

The likely difference, is that the process I described, usually took 4-6 months after the code was completed, whereas Elon Musk/Tesla can probably spin it around in 4-6 weeks.