Basically, gadgetbridge is a third party open-source application that replaces the manufacturer app for a bunch of fitness watches (and other devices of that kind).

So you can use it to replace the phone connectivity functions (like receiving notifications etc) as well as getting visualisations of the data etc. And since it all happens locally, none of your data is stored on the manufacturer’s servers. If you understand how to work with SQL and statistics, you can also run your own statistical analyses, since it’s just a sqlite DB.

The downside is that you can expect it to be limited in functionality compared to e.g. Garmin’s cloud functionality. Personally I find there’s enough data to be useful, but other’s might have different needs.