How do statups go about when building a tech product that would bring an idea to life?

I would like to know if there are some startups that open-source their entire stack. I would like to see building blocks for their web/mobile applications.

I have checked https://stackshare.io/, but it is high-level for me, I'd love to how people put code together.

Some (Small) startups I came across:

- RASA: https://github.com/RasaHQ/rasa - MapZen: https://github.com/mapzen/

Could you please share some links of an open source stack (for a product).