Did the high-level requirements resemble a concept or a document like a BRD?

High-level requirements are most often a conceptual description of the task or product, which can be documented as a concept document or as a BRD (Business Requirements Document).

If the requirements were at the concept level, it is usually a brief description of goals, main functions, and business value without detailed elaboration. Such a document helps understand what needs to be done but does not delve into technical details.

If it was a BRD, the document contained a more formalized description of requirements, including business goals, functional and non-functional requirements, constraints, acceptance criteria, etc. The BRD serves as a basis for further development and agreement with the customer.

Thus, high-level requirements can be either, depending on the project stage and methodology. In my practice, we most often started with a concept, which was then developed into a BRD.