He didnt found paypal, his company was merged with another one that became paypal. He was the CEO of PayPal early on, but was so universally disliked he was booted after 6 months.
He then took his Paypal money and did invest it in a fledgling startup named Tesla that had a working prototype of an electric sports car based on a Lotus frame. He later sued the actual founders so that he could claim he was a founder.
SpaceX is there because he had money to fund it and to entice Gwynne Shotwell, a renowed name in aeronautics, to his new space startup. Shes run it the entire time.
Musk is a "money guy" who lies a lot and has had some lucky hires. Hes a pitch man who has lived his entire life with a silver spoon in his mouth, which makes it easy to gamble on high risk projects.