Work with data/content from entries
In Craft almost everything is an element type (assets, categories, entries, global sets, matrix blocks, tags and users). If something is an element type you can attach arbitrary fields to it. Using fields like the Assets field, Entries field, Categories field or Users field you can define relations (link) between these elements.

For fetching elements via an API there are these options:


the built-in GraphQL API
the Element API plugin
creating a plugin or module and define API endpoints freestyle using the element query builders


Other non-API options for querying elements are:


use element queries in Twig and dump the result
use element queries in PHP and dump the result to the browser
use the REPL via the CLI with ./craft shell