How to create a Fitness trainer-finding app in android and ios?
I suggest you start with something simpler, like the Bluetooth heartrate monitor tutorial here: Core Bluetooth Tutorial for iOS: Heart Rate Monitor | raywenderlich.com
Once you’re familiar with how Bluetooth device discovery and connection is performed, then you can start looking at trainer-specific stuff (follow the links from that tutorial to the Bluetooth specification).
If you just want to read values, that might be enough. If you want to control the trainer, then you need to look into sending FE-C messages to the device.
There may well be some cross-platform SDK that makes this easier, but I’ve not looked.