Via Upwork, for each project. The agency contacts you, shows you the project you could work on, and then you agree on the price. It is different for every project. But once you do good work for them, you are in.

Online interview after application via a freelance platform. The team manager contacted me and asked me about my availability and experience. We agreed on the job details and the payout. And everything was good for the first job. Then i joined the team officially and had regular work.