The default Airbus lacks many important-yet-basic features which, for example, makes it unable to properly follow a flightplan entered in the MCDU (FMS).
The FlyByWire group (FBW) has made such a long list of changes, improvements and additions, that it would be quite difficult to summarize efficiently.
I think it might be interesting to check the changelog on their website. If you know well the default Airbus, then it will be easy for you to understand what kind of changes they did, or what kind of things they added. I know for sure they have made extensive changes to the aircraft model, performance, flight model, as well as the modelisation of the MCDU and other internal systems.
https://github.com/flybywiresim/a32nx/b ... LOG.md#080This addon was simply the best liner you could get in MSFS, before other makers like PMDG and such finally became active
And it's free
Concerning the updates, it will not "auto-update". However, you might need to install a new version each time the simulator received a Sim-Update.