I have been hired by a software company to develop one of their SDKs which has been open-sourced later. After this job ended I kept working with them and a some of my contributions were and are still towards open-source projects which are mostly promoted by the company themselves. I, personally, don't have experience with working fulltime on OSS but I read some stories about it where companies hire people to maintain a project they use internally similar to a SLA.

In my freetime I try to publish as much code as possible although it doesn't get much attention but without this I would not have gotten the job. For me open-source is not about getting famous or working on the next big thing but giving the community something back and not just consuming the work of others.