Here at Vively, we use a third party service called Terra. When you join Vively we'll ask you to log in using your FreeStyle Libre credentials so that we can access your sensor data.
Your credentials travel through the Terra servers and we don't get access to them, so you can rest assured they are kept securely by Terra. Once your account is linked, Terra will post updates on your CGM levels back to Vively every 5 minutes or so (provided that you scan your sensor).
This is why there can be a slight delay from when you scan your sensor with the FreeStyle Libre app and when you can see your CGM data within Vively.