Ok I have a naive question I think someone here can answer: how long do these satellites stay in orbit? Don't they need a boost every now and then to maintain the momentum? Is that boost built in?
Yes. They have built-in thrusters to maintain their orbit. If they stop working or get retired their orbit decays and they burn up in the atmosphere. See the 'ion propulsion systems' graphics at starlink.com.
These particular ones won't stay in orbit long. If left to decay naturally, maybe 5-10 years (tending towards the lower side of that range, but there's unpredictable factors that can have an impact e.g. space weather). However orbital decay times increase exponentially(ish) with your height. At the higher orbits they've gotten permission to use, it can take more like 200-1000 years to decay naturally
