You will need to pair your speed and cadence sensors to Zwift on the pairing screen. If you have a power meter, then you will only need the cadence sensor. Speed sensor will be unnecessary once you pair the power meter.
What platform are you using for Zwift (e.g., Mac, PC, iPhone, etc…)? The platform and sensor capabilities will determine if you use Bluetooth or ANT+.
If you’re using the iPhone, you will need sensors that do Bluetooth. The iPhone does not do ANT+.
Enable Bluetooth on your iPhone, then start up the Zwift app, when you get to the pairing screen you should be able to pair the sensors. If you don’t see them, you can try riding to get them active (??). But you will need to make sure they do Bluetooth. Otherwise, you will never see them and thus be unable to pair them.
The Garmin 520 itself does not have speed or cadence sensors. It can use GPS for speed or it can connect to a speed sensor. Likewise, the 520 can connect to a cadence sensor as well. Lastly, the 520 only supports ANT+ sensors.
Do you have cadence and speed sensors which connect/pair to your 520? If so, what kind?
You’ll need an ANT usb stick for your PC so the zwift app on PC gets speed and cadence data. You can get cheapo ones on auction sites YMMV or possibly better branded ones.
Your basic trainer will also need to be one “known” by zwift so that zwift can convert wheel speed to estimated power.
see https://zwift.com/hardware
You can use your iphone with the companion app when you zwift for other in game activities but it won’t be required.
Yes I do have a speed sensor as well. My next challenge no doubt will be iPhone to TV. Which I hope with a lightning cable will be pretty straightforward