What your first months of BA in McKinsey looks like?

Hello all! I recently got an offer from McKinsey for BA position and I will start in March. However I come from non traditional background (philosophy) and all the business knowledge I have is basically cases I did rigorously in prepping process. As a result I feel extremely nervous about the job itself even though I had zero problems about the job or doing the job in preparation. I saw many talented and smart candidates getting rejected and I got in. I even felt like there might be some error in the application process. Anyways my question is I don't know how to be functional for the team or what to do when I start. I know that there's an onboarding process where you get to learn essentials and so on but still I have some serious doubts. I specifically wrote McKinsey but I appreciate other MBB experiences as well. Thanks in advance!!