Except they're mostly not. While it's not hard to find exceptions most of the maintainers and other coder associated with major open source projects are being paid by companies to do so.
I do not have real data, but I assume that more than 50% of the effort invested in open source is not directly paid by a company. For the Linux kernel 20% to 30% of the commits come from people doing it as a hobby, the Linux kernel is supported very well by big cooperations.
However, in the case of the Linux kernel just about 8% were unpaid as of five years ago. https://thenewstack.io/contributes-linux-kernel/
I assume that projects like Kubernetes and OpenStack are similar.
Obviously many smaller projects are largely developed by people on their own time. I expect this also applies languages and things like that which aren't largely corporate efforts like Go and Rust are.
The thing is, while there are maintenaners of important projects working for free, the idea that most of major open source development is for free is mostly mythology. Large projects are backed by companies.