I am a backend dev and know little of web dev.

Assuming you are designing a web app, and corresponding mobile apps in 2018 from scratch, how would you go about structuring it? What would be the best practices? What tools you'd choose?

Since it's hard to discuss it without a concrete example, let's say you're designing a Quora or a microblogging site.

P.S. I am personally fond of golang.