Rewards are calculated by the network itself. CoinDCX receives the rewards from the network and transfers them to your trading account directly.
The rewards are calculated on the amount of the particular crypto you have staked on CoinDCX. The more you stake on CoinDCX, the more rewards you receive in return. Rewards also vary with the frequency of blocks produced in that crypto’s network.
To know more about the reward calculation, refer here.