The simple answer to this question would be that Meta relies on three types of data to know its audiences:
declarative data
behavioral data
and socio-demographic data
But of course, it's a bit more complex.
This data is collected on all Meta-owned apps, such as Facebook, Instagram, Messenger, etc.
The tracking pixels installed on the websites of brands and organizations which have a Facebook Advertising account are also taken into account when building these audiences. The data collected by Facebook goes beyond only the limits of the use of social networks, in fact, it comes from a global digital activity that permits a fairly complete understanding of a target audience, with its specifics and its differences.
If you want to go further, we have listed the different entry points in this article.