The redwood/packages/api and redwood/packages/web are where the package you import in your RedwoodJS project is.
Example : import { FatalErrorBoundary } from '@redwoodjs/web'
Will find the file for that in redwood/packages/web/src/components/FatalErrorBoundary.tsx
The redwood/packages/create-redwood-app/template/api and redwood/packages/create-redwood-app/template/web is all the structure and files to create a new RedwoodJS project with yarn create redwood-app my-redwood-app