Onshape and Solidworks (and Shapr3D, and Nx amongst others) use the Parasolid kernel. Without Parasolid, Onshape wouldn't have happened so quickly. It also helped that it was founded by many of the core team behind Solidworks. Parasolid started being developed in 1988. [0]
Creo uses the Granite kernel, which was developed by PTC, starting in 1985. [1]
Onshape and Solidworks (and many others) also license the same 2D solver, D-Cubed, now from Siemens as well (originally from a company founded in 1989). [0]
None of this is to say that it is impossible to start fresh, just that there are insane numbers of person-years of development work embodied in these libraries. To get anywhere near the level of completeness and power will take a lot.
Creo uses the Granite kernel, which was developed by PTC, starting in 1985. [1]
Onshape and Solidworks (and many others) also license the same 2D solver, D-Cubed, now from Siemens as well (originally from a company founded in 1989). [0]
None of this is to say that it is impossible to start fresh, just that there are insane numbers of person-years of development work embodied in these libraries. To get anywhere near the level of completeness and power will take a lot.
[0] https://www.engineering.com/story/parasolid-d-cubed-and-siem...
[1] https://www.shapr3d.com/history-of-cad/parametric-technology...