The nice thing about using 1U size chassis is that you aren't reliant on a vendor specific backplane interconnect for the blade-to-blade-chassis interface. It takes advantage of much larger economies of scale in standard AC or DC power feed interconnects and ethernet (25/50/100G ethernet). And if you buy a huge enough number of things you can get a Taiwanese motherboard OEM to design you a custom ATX (12x9.6") or full size (12x13") dual socket motherboard with all the extraneous crap stripped off, like VGA displays, USB ports, 2.5mm headphone jacks, etc.

There are "standard" widths and lengths of 1U size power supplies (Whether AC input or DC input) that are used by dozens of different system integrators. Order off the shelf based on the wattage and capabilities you need. Same thing with the chassis. At a certain quantity level you basically have a folded steel box with mounting screw holes for ATX or larger-than-ATX motherboards. With M.2 SSDs on the motherboard these days, you need only a few things:

a) a long 1U steel box with high speed 40mm fans sucking air through it

b) motherboard with cpu/heatsink/ram/storage mounted on it.

c) power supplies.