My understanding is based on the transition from 16 to 32 bit so I cant spek for 64 bit stuff specifically but its my understanding that the microprocessor circuitry for al/ah affects only the appropriate 8 bits in the physical register circuitry and that ax is likewise hardwired on the processor to affect only the lower 16 bits. I operate with the presumption that 64 bit architecture works under the same principal although I havent done much research into 64-bit systems to be honest.
A register is of course a pile of transistors laid out in such a fashion that there are 8/16/32/64 lines which represent the bits forming the value in the register. When you read/write to AL, you are reading/writing directly to the AX registers lower 8 lines and the EAX's lower 8 lines and presumably the RAX registers lower 8 lines. Like wise AX to EAX and in all likelihood EAX to RAX.
Virtualisation techniques were introduced in the 90's to allow for 16-bit processing on intel machines operating in 32-bit protected mode using v86 mode which used mapping techniques rather than switching back between real mode and protected mode since the switching incurred significant processing penalties and multi-tasking operating systems could not effectively run both 32-bit programs and 16-bit programs at the same time.
Caveat - Im unfamilliar with writing for 64-bit machines.