I think different people view answers to these questions differently, and that's because they're not all looking for the same sets of information from them. You have to try and gauge the level of detail you've been asked to provide depending on the context in which you've been asked a question.
If someone simply asks you 'why law' without any prior discussion, then go into a good amount of detail, beginning with what first sparked your interest to how you developed this interest deeper. I would not go into specifics of these details (dates, background etc, challenges etc.) unless prompted further. If you've have had conversations around 'why law' or 'why this firm', then add detail as needed.
Having a solid structure ensures that you add an appropriate level of detail and that your answer flows well into any follow-up questions. I would have at least three well-developed points ready for questions like 'why law'/'why commercial law', 'why this firm' and 'why you'.
For why law, it could be something like what first interested you in law (an event, something you read, a conversation etc.) ➡️ the steps you took to develop your interest in law as a career (extracurriculars, academic initiatives, attending career events etc.) ➡️ why you think the law is well suited to your skills, experience and character traits (pick an example, analyse how it would help you as a layer and link it to a career in law)
For why this firm, you could try stating XYZ thing you liked about the firm ➡️ how you learnt about this (research, attending an event, speaking to someone from the firm etc.) ➡️ how you know this is important to you (link back to things you want for yourself in your training contract, your ambitions for your career etc.)
I hope this helps?