Hi,
You might want to look at the External Functions API (https://docs.moodle.org/dev/External_functions_API) and less likely the Web Services API (https://docs.moodle.org/dev/Web_services_API). You might also want to read the 'Creating a web service client' page at https://docs.moodle.org/dev/Creating_a_web_service_client. Moodle, out of the box, provides a 'core_user_create_users' and a 'enrol_manual_enrol_users' web service functions both documented on the Site administration->Plugins->Web services->API Documentation (/admin/webservice/documentation.php) page.
You might want to look at the External Functions API (https://docs.moodle.org/dev/External_functions_API) and less likely the Web Services API (https://docs.moodle.org/dev/Web_services_API). You might also want to read the 'Creating a web service client' page at https://docs.moodle.org/dev/Creating_a_web_service_client. Moodle, out of the box, provides a 'core_user_create_users' and a 'enrol_manual_enrol_users' web service functions both documented on the Site administration->Plugins->Web services->API Documentation (/admin/webservice/documentation.php) page.
As a side note and at the risk of sounding mercenary, my book 'Introduction to Moodle 3.9+ Plugin Development' (https://mukudu.net/publishing/m39plugindevelopment/index.php) contains 2 examples of developing external (web) services in Moodle as well as another 8 plugins.
Have fun!