Also while NAND mirroring does work, there is a good chance it wasn't used in that case, the current "theory" is that one of the patched USB DMA exploits were used to unlock the phone, NAND mirroring was too "risky" for the FBI to consider at the time, and it wasn't forensically secure IIRC one of the reasons for the very high price was the non-physical attack approach that the 3rd party provider they've selected had in their toolkit.
Cellebrite has a few of them I know of a few for MediaTek chipsets that work through the USB host and the camera connector.
This isn't any different than the DMA "skeleton keys" for PCs and Macs that work via firewire/thunderbolt/pcie/expresscard/PCMCIA etc.
https://news.ycombinator.com/item?id=10906999
Mobile phones aren't secure. Period. Anyone saying otherwise about theirs better provide convincing evidence that they mitigated everything on that non-comprehensive list. Then we talk about what I left off. When FBI said they couldn't get in, I knew they were lying to try to establish a precedent & technical method to increase their convenience. It was also only time Richard Clarke's pronouncement was relieving where he said (paraphrased) "I'd just ask the NSA to break into it." Because they or a contractor could due to risks not mitigated. Obvious.
Honestly not sure if I'll ever trust a tiny, slim smartphone for high-security INFOSEC given the effect of EMSEC on size and hardware POLA on power consumption. If their physical specs are Apple-grade, then they cut corners on INFOSEC and some government hacker has job security.
[0] https://youarenotsosmart.com/2011/06/10/the-backfire-effect/