Yes, the synthetic vision feature is provided by the iFly software. But it needs input data from sensors to work.
The best option is to get input data from an AHRS, or Attitude and Heading Reference System. Many ADSB devices include an AHRS sensor package, and provide the data from those sensors in a standard format that iFly can consume to drive the synthetic vision. The sensors provide direct data on aircraft roll, pitch, and yaw (RPY), and maybe other data, depending on the AHRS device in question. (iFly doesn't necessarily use *all* of the data provided by an AHRS, but it definitely uses the RPY info.
If AHRS input data are not available, iFly can use alternate means to drive the synthetic vision. It will use several consecutive GPS position readings to determine your path through space, then do math on that set of data to derive your pitch, roll, and yaw. For instance, it will assume you are pitched 0deg if you are flying level, or that your pitch matches your flightpath climb or descent angle if your altitude is changing. It does similar math to derive roll and yaw. However, these are simplistic approximations. For instance, your pitch will be very different at max cruise speed vs. stall speed, but if you are maintaining a constant altitdue, iFly will show you at a 0deg pitch angle for both flight conditions if it's using GPS data. An AHRS would not be fooled, and would sense your actual pitch angle, so iFly would show you pitched higher up at stall speed than at cruise speed.
Similarly, you could be in a slip, with the aircraft yawed and rolled in a cross-controlled way to maintain a straight-ahead flight path. If iFly is using GPS, it will show you with 0 roll and 0 yaw, but if it were using AHRS data it would show the correct roll and yaw.