It's different depending on whether the job was a direct or recruiter job, of course.
Once you and the client decide to work together, our job is (mostly) done. We are always available if problems should arise, but apart from that everything happens directly between you and the client from then on.
The contract is signed between you and the client directly. Please make sure it is legally solid and, if you are unsure, prefer a Dienstvertrag to a Werkvertrag. If the client asks you for a contract, feel free to use our template for a Dienstvertrag.
You issue your invoices for your work directly to the client as well. Please forward each invoice to us (preferable in the corresponding Slack channel) as soon as you sent it to the client, so we can send you an invoice back for our fee.
The manner of working together is quite different when it comes to recruiter jobs.
You will sign a contract with the recruiter which details the arrangement of working together. In rare cases, the recruiter will ask you to sign a contract with the end customer instead.
You will most likely submit a timesheet at the end of each month, and the recruiter will pay you the previously agreed upon hourly/daily rate once the timesheet has been confirmed by the end customer.