You can find some more general advice on structuring/explaining "why this firm" here
https://www.thecorporatelawacademy.com/forum/threads/explaining-interest-in-firm.3079/
In terms of the level of detail you'd need, you'd really need to be demonstrating that you've done your research into what makes those departments high ranking/innovative. What kind of disputes do their litigation team specialise in that sets them apart from competitors? What kind of services do they offer that make them innovative?
With regards to the pro bono point, I agree with Karin on the point that for DLA it might be slightly more acceptable since they do have such a strong emphasis on it. But I would still recommend picking one/two projects in particular that they have organised that resonate with you and most importantly explaining why they resonated with you and why their emphasis on pro bono is important to you as an individual.