They work with Clevo to develop them[3], and make the hardware work with Linux[0], preferably in firmware (so you don't generally need the System76 driver). They also set them up with CoreBoot and open source EC[1].
Although Clevo can sell their own version of the hardware, it is not really the same[2]
[0] Based on my experience. I ordered a laptop a long time ago and they explained they were working with Clevo to get the firmware fixed so that suspending worked reliably. This is before the OpenBoot + EC stuff came about.
[3] Clevo is System76's ODM. Most OEMs (e.g. Dell, HP) work with ODMs to put together the final product the OEM then offers. I posted the public Dell ODM list in another thread. System76 wanted to bring laptops in-house; I don't know the status on that. But System76 and Clevo work together to offer the final laptop based in a Clevo base[4]. And now (like, in the last 4 years) System76 started also adding open source firmware in some places.
This is manifestly not the case. Perhaps in the very early days but not in the last several years.