CGMs use the interstitial fluid in (usually, although people also use other spots) inside side of the upper arm to measure blood sugar. As long as they are placed in the right spot (i.e. in a fattier area instead of directly on top of muscle, hence why it is on the inside side of the upper arm and not the outside) and the sensor doesn't fail, they will measure blood sugar fairly accurately for 10-15 days depending on the CGM model (disclaimer: They do tend to run approximately 15 minutes behind the number that an actual finger poke would show you).
Nightscout is an awesome tool because it can be self-hosted so that you keep full control over your health data, but since it is a web application, you can still easily share it with other people too, like family or your endocrinologist.
On android, you can use the xdrip+ application to pair with most types of CGMs to completely keep your data out of the cloud, or you can also pair it with the API from Abbott or Dexcom (depending on the CGM model) to pull data from their 'share/link' applications, if you opt to pair with/use their official apps
[0]: https://nightscout.github.io/ [1]: https://github.com/nightscoutfoundation/xdrip
I personally use Dexcom G7 15 day sensors (because it is what my insurance covers) and use xdrip+ to upload the data to a nightscout instance hosted in my homelab