I am a BA in a company that provides insurance solutions to clients.
We work in a hybrid methodology between waterfall and agile (project divided into Sprints, each Sprint will go as a waterfall life cycle).
Now I know that the solution Specification document should be prepared by myself, but I'm not sure about the BRD.
Whose responsibility is writing a business requirements document (BRD)? Should it be from the client side or vendor side?
The team as a whole (developers and client) should take responsibility for creating any documentation. It's a collaborative effort and to maximise productivity the team should try to avoid having unnecessary hand-overs of documentation between different sub-teams.
If your sprints have a "waterfall lifecycle" then they are not sprints. Agility is not essentially about doing work in iterations but has more to do with working collaboratively to solve problems, responding to business needs and minimising bureaucracy and hierarchy. The fact that you are asking this question suggests that you are open to some alternatives, so maybe you can raise your question with the rest of the team before deciding how to proceed.
In my practical experience, the truest answer is: "both of them." Because, in the end, "the solution" consists of computer software. Which basically means: "in the end, 'the digital computer' (a purely-electromechanical thing ...) will out."
The inevitable solution consists simultaneously of both "what the customer wants" (client), and "what the [undoubtedly ...] already-existing platform can right-now do."
Therefore, strive to engage both interested parties at once. And, in doing so, do not pretend to yourself possess "the necessary technical expertise." Rather, your role is to be a facilitator. Your immediate task is not "to do it [yourself]," but instead "to be sure that they do it."
Your question implies inherent righteousness in the development of this product. There is none of that. If the client has the internal expertise to develop it, and they choose to make it instead of buying it, then it's in the client's scope. If they choose to buy it, then it's in your scope.
The owner of the requirements is the client. We expect them to prepare the BRD. But if they did not already prepare it and attach it as an addendum/annexure, it means that they are not capable of doing it, and they expect the vendor to prepare it.
In the end, you need a solid BRD. If the client can not do it, someone has to do it, and that "someone" will probably be from the vendor side, unfortunately.
