Framework does work with ODMs (Compal, I believe, is their main one?) to design mainboards for their chassis, which are designed specifically for Framework. It's not like they just take an off-the-shelf design and build it without any modifications.
And yes, chipsets change. (A "socket" changing isn't really a thing when we're talking about a laptop where the CPU/SoC is soldered in.) Generally this isn't a problem, though: as long as you can design something that physically fits in the chassis and supports the features you want, you're fine.
