Multi-Manager Scenario-Driven Modelling

This is mainly a question on model-setup.

Most of the models I've been exposed to are sell-side, which I've found overly complex and extremely difficult to (1) set-up probabilistic expected value and really vigorously model out scenarios and (2) distinguish between short- and long-term drivers.

I know the multi-managers are really scenario-driven and obviously have quite detailed models.

How do MM analysts structure models so that they can run detailed scenarios (not simple upside/downside) and still have 2,000+ line models?

Any advice would be appreciated.

Cheers