Did Amazon, Facebook, Netflix, Instagram or Whatsapp (and other big companies) scrapped their initial startup code and started from scratch at some point?

My question is related with my yesterdays post about how (and if) to refactor 15y old codebase.

Let me rephrase the question a little. Considering big, successful companies, I imagine they started with code that was not great and at some point of time they faced situation that the code maintenance cost was too expensive, adding new features was a pain and scaling team size was not doable because of dependencies.

Did these companies followed the path of huge refactor on existing code, piece by piece? Or rather part of team was maintaining old system, when other were developing new system?

In this post I am NOT asking about “refactoring theory” or best approaches…. but I am asking about if you know a real use case? Or maybe all that big companies still have part of their codebase a crap and nobody dares to touch it?

I hope many of us here will benefit from the discussion.