Hey Max974,
I would first start with reviewing the documentation of a general overview of Enviroment Query System Overview Link, much of the functionality in blueprint is mirrored in the C++ API Documentation, however, reviewing the general system and how it communicates through blueprint nodes may be of some use to you in your C++ implementation. I believe you may also be interested in Query Result, if you intend to implement your own EQS Test functionality.
Please let me know if you need any further explanation or additional questions.
Thanks