Simple, you usually don't need that. Most devs now a days don't know how to build efficient websites. That's why they need a tech Stack like the Mount Everest and create a dependency hell. It was possible to serve more then 250k unique users per day from a single VM more than 10y ago on shitty hardware. You can deliver so much more on modern tech if you stick to KISS principle.

But yes, some rare sites do need to scale above >1M unique users, that's where the trouble begins. You can start by benchmarking everything you build. Think of every line of code and optimize it, or you just build your stack in smaller pieces and scale out using modern tech Stack. The question is, what do you want to learn. The first way would learn you how to build efficient software, the second one would teach you how to waste lots of resources but be new, modern and a hipster. And in addition, now a days employees are so expensive, that a company don't care about Cloud costs if this makes dev faster.