Hi @bashox,
With regards to the WooCommerce REST API access codes for that can be set from inside your WooCommerce site by following this documentation.
https://docs.woocommerce.com/document/woocommerce-rest-api/
For the developer level documentation, you can find located on this site below and this covers authentication and all of the endpoints and parameters which can be accessed via the REST API.
https://woocommerce.github.io/woocommerce-rest-api-docs/#introduction
With regards to testing API calls, we have some information on testing API calls using an App called Postman and I’ve referenced that for you below too.
https://github.com/woocommerce/woocommerce/wiki/Getting-started-with-the-REST-API
With regards to integrating plugins into the WooCommerce rest API if those plugins do not already support the API the developers of those would have to update the to integrate with the API.