If it's a commercial project devs will be paid because they're like employees. If it's a small team making the project they won't have fixed salary. There's going to be donations buttons & donations links so devs can receive money from ppl who appreciate their work.
Unless the open-source product is produced by a company that has a relatively steady cash flow and a fixed amount of employees, then getting a salary isn't guaranteed. In fact, it's the opposite!
Are there open-source products produced by companies that pay their devs? Of course! And you can also contribute in these products, but if you 're not in their payroll, you will not get a salary.