It depends. Many are paid as career OSS developers to contribute to Kubernetes.

The ingress-nginx maintainer just stepped down today because nobody would fund his work. https://twitter.com/aledbf/status/1347273455448842240?s=20

openfaas is also unfunded and a large project with many commercial end-(ab)users. https://www.openfaas.com

Matt Holt behind Caddy gave up after years of struggle and managed to sell out to a company.

My take on OSS maintainers - https://blog.alexellis.io/the-5-pressures-of-leadership/ - see "2. Pay"

It depends on the project. Some projects (can't think of examples off the top of my head, sorry, but they have come up here on HN) do reach pretty widespread usage without anyone being paid to work on them full time, and potentially with only one unpaid developer on the project.