Ask HN: Outsourcing front end?

I hear that many (young) startups outsource their frontend coding. Can anyone give me some advice on how they integrate those frontends? I'm interested in the workflow between frontend devs and the backend app.

They work on different repos? The frontend guys work directly with the template engine? Or they deliver pure HTML and a backend dev makes the integration? Do the outsourced frontend devs have full access to the app's code?

Any input is greatly appreciated. Thanks!