@BarbourSmith a very good question.
In V2, the best approach is to make a project folder. And then organize the parts as desired, including sub-directories.
project/
- index.js // this file is the entry point, and exports main
- tube.js
- subassembly/
- index.js
By the way, if the project works with the CLI, then the same project should work with the WEB UI as well.