>Or do have these Fossify application some extra privacy features that do not come with the default ones?
It's this one. Google's built-in apps are closed source with undisclosed telemetry. Fossify are open source, and they don't send your contacts or calendar entries to Google. Google's apps also serve Google ecosystem lock-in, and Google's ecosystem serves ads.
The non-google options on the Play store give you the option of not sending telemetry to Google, but at the cost of typically violating privacy in other ways or including ads.
Fossify avoids either of those costs.