TexAgs91 said:
As of just after separation, looks like Starship already has a leak (see the bottom video)
or is that an RCS jet?
That's possible but unlikely. Since the ship separated using hot staging, the ship's engines were burning at that moment. I have not seen them use RCS thrusters during a burn. It is more likely the ship's autogenous pressurization system vent.
Both stages use ullage gas pressure to feed the engine turbo pump inlets. Basically using its own cryogenic boil off to pressurize the tanks. Once the engines light up they burn a literal ton of propellant a second. So the pressure in the tanks will begin to drop if not replaced with more ullage gas. So the autogenous pressurization system uses a pump cycle that uses heat from the engines to vaporize the liquid propellant and flow it back to the tank to maintain the ullage pressure as the propellant is consumed during burns. Since the system will surge pressures right after engine start, there is a venting that happens to release the surging pressure to help it reach a stable pressure quickly. That looks like that vent. You can see a similar vent on the booster right as it start lifting off.