Do you believe the vast majority of investment happens because someone like Elon Musk personally goes around to give every little venture money?

Here's a non-exhaustive list of ways the wealth of multiple people can be pooled: creating a joint account, investment clubs, general/limited partnerships, LLCs, crowdfunding platforms, and syndicates (you're on the website of one right now).

There's also government-backed pooling/funding vehicles and banks engaging in various forms of investment, funding, loans. Any existing corporation is also capable of investing themselves.