She uses xDrip on her Pixel 3XL with a Dexcom G6 as the collector which feeds into Nightscout directly. Then the OH Rule above polls Nightscout’s API for the most recent reading, parses the JSON returned, and updates a few Items with the values. In this case, the time of the last reading (NS_LastReading_Time), the last reading (NS_LastReading), and the trend (NS_Trend).
I’ve self hosted Nightscout on my LAN and we gain access to it through VPN.
I mainly coded this so I could incorporate the reading/trend with the automation (e.g. turn a light red if there is a low or trending double arrows down) but haven’t really done anything with them. When she wants to see the readings it’s on her watch or on her phone. When I need to see the readings I can bring up Nightscout directly or set up xDrip on my phone as a follower. I have these Items on my sitemap but I don’t actually use them for anything there. They are mainly there for testing purposes to verify they are being updated.
See NightScout openHAB Integration for more details on the Nightscout integration.