Tools and approaches for maintaining and enforcing production quality code

I have a number of friends who are Tech Leads.

Was really interesting to hear how they work towards maintaining production quality code.

I realised that there are some common, low hanging fruit that we all had, like Contributing Guidelines, and then more intricate 3rd party softwares I had never heard of.

So my question to the Tech Leads here, and those aspiring to be Tech Leads (and of course everyone else!) in this subreddit is this:

If you were at a tech startup and you were their first tech lead (hopefully backend but frontend contributions are fine here as well), how would you setup the development process so that it was built on solid foundations, ensuring that quality code only ever ends up in production and all code is documented and easily debuggable. I want to know your tools, frameworks, GitHub integrations, documentation, everything you think would help in this cause!