Hi Benjamin,
external systems and apps need to use WebServices to communicate with Moodle. You can see the list of WebServices available in your Moodle site in here:
Site administration > Plugins > Web Services > API Documentation
Then you just need to perform a network request to this URL:
{YOURMOODLE}/webservice/rest/server.php
With the params:
wsfunction: Name of the WebService you want to use. For example, core_course_get_courses.
wstoken: User token. You need to get it from login/token.php using a login form.
Other params required by the WebService.
Dani