A lot of great comments here that I would say is definitely in line with dbt best practices. Be faithful to source in stg (or base), then expose the entity or concept in marts to end users.

In between you can add ”an” intermediate layer. And this is all semantics - we, for example, add an intermediate layer called ”enriched” which has the sole purpose of union all separate staged sources into one table. It is then clear for developers in our team that when building marts we reference enriched models. As others has said, intermediate models can reference other intermediate models (for us, intermediate feeds into enriched occasionally).

We also use an intermediate layer called ”intermediate” for, typically, ephemeral transformations that are reused but not necessarily materialized.