Hi George,
1. The ANT+ logo means that the device has been ANT+ Certified, thus it should be present in the ANT+ Certified Product Directory where you can determine which Device Profile it uses.
ANT+ is common on fitness machines to either display sensor data such as HR from the strap so you don't need to bring out your phone, or to broadcast the Fitness Equipment Device Profile, which allows you to save workout data from the session to your personal device (phone, watch, etc) or for the gym to collect that data for you.
2. If the manufacturer doesn't specify you'll just have to read the device profile to see which data fields are required/optional, though this doesn't include manufacturer specific data they may have also included.
3. Technically ANT+ device profiles are mostly broadcast based, meaning you can collect the data using any number of devices concurrently, as in your watch or phone could be recording the data at the same time.
4. iOS doesn't enable ANT+, even if it's present on the chip. Whereas many Android manufacturers enable ANT+ either natively (Samsung, Sony, etc), have the hardware present to enable it (OnePlus, Xiaomi, etc) after installing some apps, or support the USB Host API which means you can plug an ANT USB Stick in and use it as a dongle.