Musk started SpaceX with Michael D. Griffin, the guy who invented large constellations of military satellites to win a nuclear war. And then he funded Starlink.
Starlink project began after Musk and Greg Wyler parted their ways. Wyler approached SpaceX in 2014 with a proposal to build OneWeb (then called WorldVu), and initially they worked on the project together. But then they started to accuse each other of doing various underhanded things, and split. After that, Musk decided that he could do a similar and even better system without Wyler, and that's how Starlink was born in 2015.