Once you have a reference to a TBluetoothLEDevice instance, you can discover the services exposed by the device in order to get access to a TBluetoothGattService instance. This instance represents the service you are looking for and will get you access to the underlying characteristics.
Once again, the model is asynchronous so you will call Connect on the TBluetoothLEDevice instance and then start the discovery of its services with a call to the DiscoverServices method. Then the TBlutoothLE.OnServicesDiscovered event will fire when the operation completes. A convenient argument of the AServiceList event handler, TBluetoothGattServiceList will let you iterate through the discovered services and, for each service, you can enumerate its characteristics (through the homonym property).
A reference to the characteristic is needed to read and write data, using the TBluetoothLEDevice.ReadCharacteristic and WriteCharacteristic methods, as...