The Other Half (TOH) is a swappable back cover that does more than protect your phone — it transforms it. The original Jolla Phone (2013) pioneered this with open I2C interface and NFC-enabled covers that changed themes and behaviour just by snapping on
(…)
With the new Jolla Phone, we’re taking TOH even further — and we’ll open source the hardware and software interface specs so anyone can design, 3D-print, or produce their own modules. Primarily we plan that the new The Other Half interface would be based on I³C.



Or they admit that it contains “some proprietary components” but don’t really elaborate, perhaps because they themselves do not know which components those are. Still, the insinuation is that it’s mostly free or that the proprietary bits are so insignificant they do not matter, which I’m skeptical of.