Relational Database.

Just use Postgres really.

Here you go, one less thing to worry about. Thank me later.

Maybe in the future when you actually start having problems, then you can consider options to solve specific problems. But I don't think the concept of "choosing a database" is really useful.

If Postgres is enough, why do all other databases still exist? I've never used Postgres so I don't know much about it.

My guess is that the point they are trying to make is “introduce complexity when it appears”. The data access patterns that users need aren’t always obvious, at least in my experience.