For what type of data it's better to use relational, and for what type of data, non-relational databases?

I am trying to write my first big backend project. This is a mobile/web application like Instagram but for different purposes. As I searched through the internet I found that Instagram uses PostgreSQL and Cassandra as it's main databases. But I don't know for what purpose/type/part of data it uses which database? Does anyone know more about the databases Instagram use or in general, may I know how to decide for what services or what type of data/application it's better to use SQL or NoSQL databases?