What are your tips for building a scalable Node API?

I'm going to create an app. For simplicity, let's say people can register and then connect to each other, and even non-registered people (think family tree connections). Next to that, anyone can upload images and documents and create notes and quotes for each connected person.

I want it to be scalable of course. For learning purpose, I want to create a Node API and a responsive JS frontend (probably React). I'd like the app to be able to grow to potentially hundred thousands of people, in the database.

I'm thinking a REST API on Express and MongoDB. But that's because it is familiar to me. Any tips on which Node frameworks, which databases? And for the latter, how to structure the database? Make use of cross tables, or not at all, etc.? Love to hear your input!