I would suggest to start first with "I dont know how app works" and get through nRF5 Getting Started section, possibly also through Tutorials on this page.
Sounds to me like you are looking for someone who will develop the solution based on your requirements, so more recruitment question then technical one for this forum;) nRF Connect can act as both GAP Central and Peripheral (depending on underlying Android or iOS API and HW capabilities), the same applies to GATT Server and Client roles, but as a tool it's generic app. If you want something specific for bidirectional data transfer then Nordic UART Service (NUS) examples are the right thing for you. Thee are two examples in nRF5 SDK (GAP Peripheral and Central) and there is mobile app in Play Store which knows the protocol and you should be able to connect to nRF5x device running that example FW and retrieve some data from it (if it sends any;).
Sounds to me like you are looking for someone who will develop the solution based on your requirements, so more recruitment question then technical one for this forum;) nRF Connect can act as both GAP Central and Peripheral (depending on underlying Android or iOS API and HW capabilities), the same applies to GATT Server and Client roles, but as a tool it's generic app. If you want something specific for bidirectional data transfer then Nordic UART Service (NUS) examples are the right thing for you. Thee are two examples in nRF5 SDK (GAP Peripheral and Central) and there is mobile app in Play Store which knows the protocol and you should be able to connect to nRF5x device running that example FW and retrieve some data from it (if it sends any;).