I'm in the embedded industry and frequently look for new parts to use in designs and I can tell you that whenever I come across a part that requires an NDA to get a datasheet I skip that manufacturer and look elsewhere. Unless someone has a part so unique that there's no other choice I won't give manufacturers who demand an NDA the time of day.
It works both ways . the fact that the designs you do can be done without requiring NDA's makes so that the manufacturer of such parts also wouldn't give you the light of day.
The same goes for products that require certification ( usb , wifi, bluetooth etc.. ) You have to pay to get the specs and the certification. If you can't handle that :buy a ready made pre-certified module.
Wanna do anything that is life-impacting ? can't use any standard part as none of em are approved for such use.. Need such parts ? sign on the dotted line please ...
Other scenarios : Complex SOC with 'tilt bits'. Case in point : raspberry pi. Why is that not open source ? becasue they cant ! The development of such silicon takes a fair amount of money. The maskset is horrifically expensive. So , we will spend a little bit more and bake in a bunch of features that can be disabled using tilt bits. Same thing for the boot system. if we find issues post-mask ( things we could not fully cover in simulation ) we can apply tilt and disable or re-route items. This is part of the internal household of the manufacturer/ designer.
so with 1 mask they can make 2 or 3 devices. Depending on the tilt bits you get one or the other. or a different speed grade.
Some processors are available with different memory sizes. Do you really think that are different masks ? Same mask. the memory bits are programmed during test to mask off the banks. The flash is partitioned in multiple banks. they can alter the mapping in hardware. if there is a bunc of corrupt cells in bottom half , they swap with top half and sell it as the lower memory version. There isnothing wrong with the chip. just speck of dust on the mask killed some of the memory. mark it disabled and move on ...
There is no chance you will ever find out how that system works and what all is possible. the Raspberry pi needs to boot from a pre-packaged binary that contains bugfixes , mappings and features. that boot sauce sets up the devices and then seals it. you can't start without that package. The package is built for your 'capabiliteis set'. It is highly likely the same processor can be sold with more memory , faster speed , and more features just by altering the bootcode. But you will never get to know. Even an NDA there will not disclose how that works.