This really depends on the project. Many people just do it in their spare time and have a normal software engineer or similar role working on proprietary software using open source. Some are still at university and studying. Some are contracting and probably also get some new customers thorough their open source work.
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.
