It's called a "proc fee" brokers typically get about .35% of the loan amount but that can vary.
I'm guessing based on that they don't earn a salary - it's purely commission based. As .35% on a starter mortgage you'd be looking at least 1k commission. No idea whether they have to split that commission with the platform they work on, but putting through say 10 a month which doesn't sound like that much work is then 10k a month.
We used hey habito for our first mortgage and you basically fill in some details and they put you in touch with a broker/advisor so it's not like they even have to go searching for business.