Is this same method employed across different generations of iPhones?
Thanks
Basically, the Secure Enclave contains a 256-bit AES key physically fused into the silicon during the chip fabrication process. Apple don't know this key, and neither do the manufacturers. It's different on every iPhone. The key cannot be read by any software, or the OS, or even firmware. All that can be seen is the result of using it in a crypto operation.
The key used for actual encryption on iOS is derived by taking an intermediate key derived from the PIN, and then entangling it with the Secure Enclave key (and, I believe, the CPU's key, which is also unique and fused into the hardware, but not quite so secretive). This effectively ties the crypto process to the phone - if you take a data dump of storage and try to brute force it on some more powerful kit, cracking the PIN isn't enough. You'll also have to crack both the AES keys.
This isn't universal across all iPhones - I think the 5S onwards have it.
A lot of the conversation happening in regards to this FBI case is only due to it being an older iPhone that doesn't have this special hardware. Which is why people are confused with the FBI chose this case as their poster child when it would have made a lot more sense with the most recent iPhone.