Within a stack, can I organize pieces into files or folders underneath?

Absolutely. You can use whatever techniques you would use to organize your Node.js or Python applications to organize your Pulumi projects written in those languages. It’s common to factor infrastructure into components, and put components into separate files and folders, and then to import these components into a main

index.ts

file that composed together your final collection of infrastructure.

Awesome. I was trying to find an example, but haven't found one.