There are plans now in 2025 to work on this slowly. A few apps have recently been opened up. More are coming. So it is underway.

In 2015 Jolla were bought by Russian owners. They didn't understand open source or free software, they just wanted something for the Russian market.

In 2021 these ties were broken, but it took a long time since the Russian owners didn't respond in any way. It is only two years or there about that they are on their own feet again. They are still severely understaffed.

Don't know 100% sure. But would dare to claim most UI apps are still closed source. All the basic libraries and probably most daemons are open source. In the HW adaptation it looks bad again, but there Sailfish is not to blame.

They've been opening up bit by bit. First stuff like 'jolla-weather', recently, the notes app and numerous bits in the backend ... Currently sync for nextcloud system integration is in the works.

Jolla made a lot of promises over the years....

Not openly published but they will send it to you on request.