When a signup needs a human, you take the keyboard. When you are done, the automation picks up where you left off.
The signup asks for you when it meets something it should not decide alone — a challenge built to require a person, an unexpected page, a step that needs your judgement. It waits rather than guessing.
Open the live view and take over. Your keyboard and pointer input travels over the authenticated API channel to the worker, which applies it to the real browser window. You are driving the same session the automation was driving.
There is no "done" button to hunt for. Once the blocking step is cleared, the automation resumes on its own — it is watching the same page you are.
Handoffs happen when you are not at a desk. Touch input is translated for you, and a signup can reach you through a short-lived link. See Handoff on your phone.
Some challenges cannot be cleared from where you are — a service may want a different network, or may have decided against the signup entirely. Abandoning the job is a legitimate outcome. See When a signup fails.