There are multiple endpoints for doing selections in the Engine API. Which one to use will depend heavily on your specific requirements, but the selection in the Qlik Sense client are performed through the selection methods for list objects and hypercubes in the GenericObject class. For instance this one that allows you to select a set of cells in a hypercube:
https://help.qlik.com/en-US/sense-developer/February2021/APIs/EngineAPI/services-GenericObject-Selec...
Then there are selection methods on Fields as well, but they typically provide less granularity compared to the generic object methods. This one, for instance, allows you to select a set of field values:
https://help.qlik.com/en-US/sense-developer/February2021/APIs/EngineAPI/services-Field-SelectValues....
