A lot of laptops use proprietary model-specific motherboards - the mobo is often the first thing a manufacturer can reshape and redesign so that the overall dimensions (and price, and capabilities) of the laptop is what they want to offer.

I've seen a lot of laptops use modified "FlexATX" form factors - these would fit into pretty much any chassis would could accomodate standard ATX. But they wouldn't be compatible with ATX-based PSUs, laptops invariably use battery power and often integrate the charging circuitry onto the mobo (or onto a modular logic board which uses some non-ATX power connection to the motherboard). Mobile GPUs have their own form factors as well, laptop mobos can't accept normal PCIe hardware. If you really want to get into the hardware/electrical side of things then you could conceivably fit any random mainboard from any kind of mobile computer into any kind of chassis large enough to hold it.

What you want to accomplish is not impossible (and many people have done this sort of thing with many different mobile devices). But you should first narrow yourself to a particular laptop model or chassis style or other particular hardware requirements before digging for deeper details. Short answer is "it depends" on too many different proprietary variations and on the hardware "standards" which have evolved in two different PC industries.