de0u Every ARM SoC is proprietary. They can have open source components but the ARM CPUs will be closed source, and in practice most of the hardware and firmware is closed source. There's no alternative available.
On the Pixel 6 and later, the SoC doesn't include any radios but rather there's a separate Broadcom Wi-Fi/Bluetooth SoC, a separate Samsung cellular SoC and a separate Broadcom GNSS (GPS, GLONASS, etc.) SoC. On both the Pixel 6 and later with this design and the earlier Snapdragon Pixels with a single baseband for each of those radios included on the SoC, those components are isolated. GPU and other similar components are also isolated.
Pixels use Trusty OS as their TrustZone firmware implementation, meaning it's based on an open source project. They also run Trusty OS on their secure core, which exists mainly to communicate with the separate secure element SoC. The secure element is a separate RISC-V SoC based on OpenTitan much more hardened against both software and hardware attacks than TrustZone can be, since TrustZone is only a standard ARM CPU mode on the regular CPU cores rather than dedicated hardware hardened against attacks. TrustZone has been largely replaced by the secure element functionality and other functionality is in the process of being replaced with virtualization. The main remaining use for TrustZone is that due to being a CPU mode, it's a lot faster to use it than communicating with a separate chip. There are still 2 separate hardware keystore implementations: StrongBox HSM provided by the secure element chip and the traditional keystore provided by TrustZone. The TrustZone implementation is much lower latency and higher throughput since it runs on the main CPU cores, and it doesn't have limited data since it stores the data encrypted with a hardware bound key via an OS service with a system for rollback protection.