Hello codecast
To use the REST API plugin, you have to make HTTP requests (and include the JWT token in the X-Authorization header; you must first get the JWT token using the user token).
There is an example client (accessible from the plugin "Configuration" screen) which makes GET requests.
It allows you to browse the API routes, the verbs and parameters.
General help on how to use the API can be found here.
Typically, to import data, that is to INSERT into the database, you will make POST requests.
To test the API, Postman is a really popular client.
Bruno is a good open source alternative I would recommend.
Then, you will have to write your own API client, in the language you prefer.
Do you have or know someone with the required skills for this job?