Are you referring to personal-computer (PC) motherboards, or general motherboard (incl. mechanical design) concepts involving e.g. mezzanine and similar expandable/stackable-card system design?
Intel used to provide reference designs for their commercial PC chipsets - I used them years ago for helping debug with a PCI add-in card development project.
But, it seems they are not available for chipsets after mid-2000s, or perhaps you must be a registered developer nowadays (with NDA and so on).
If you search for "motherboard chipset reference design schematic" you will find a number of such Intel chipset/motherboard schematics, but they all date around 2003 or so.
Maybe Nvidia or other chipset companies are more open - not sure.