If you get the job, what will you do in your first week?
Suggested answer
The interviewer wants to know whether you have a realistic plan for the early period, whether you put learning how the team works first, and whether you avoid promising things that aren't possible.
A good answer breaks down into a few specific goals, instead of saying something general like "I will learn a lot."
First, get the project running on your own machine within the first few days, and write down the places where the setup instructions are missing or wrong. A newcomer sees the snags that long-time members no longer notice, and fixing them helps the whole team.
Second, understand how the team works: the branch naming convention, how many people need to approve a pull request, which automated checks must be green, who reviews. A quick way is to read recently closed pull requests.
Third, take a first small task, such as fixing documentation or a small bug that has already been picked out, to learn the process without the pressure of a hard problem.
Fourth, ask your mentor for a mid-point feedback session, and ask what they expect from you at each milestone of the probation period. You can also say that you keep a short end-of-day journal to note what you didn't understand and what you tried before asking.
Bonus points: mention that probation is counted in calendar days but contains fewer working days, so goals need to be the right size, and that you don't need to know the whole codebase in your first week.