> If we're pushed to delegate most work to them, how do you build that expertise?

Have you ever pair programmed with someone? It's the same idea. You can be an active enough participant in the process if you wish to be and can be just as knowledgeable even if some of that knowledge lies in transactive memory. https://en.wikipedia.org/wiki/Transactive_memory As long as you have the map, and the map to the map, you don't need to retain every fact about the landscape.