I got a little more info @jan in case someone looks into this.
The CONTRIBUTING.md does not mention you need to start the app as normal npm run start and additionally run webpack-dev-server using npm run dev. I assumed as written that the dev script would also creating a running instance of n8n.
In discovering this though, I also found a weird issue whereby I experienced the same missing sqllite3 error as here Getting an error about SQLite when attempting to run after fresh install - #10 by jan when running:
$ lerna bootstrap --hoist
$ npm run build
$ npm run dev
$ npm run start
Regardless if I left the dev server running or not, I could not get it to initiate the start script and I verified sqllite3 was installed in node_modules from the bootstrap command. However, when I started with a fresh pull and re-ran with start before dev it all worked no problem.
Hopefully this is helpful to anyone else who stumbles along trying to develop. I’m using OS X 10.15.7, node 12.14.1, python 2.7.16