Copy it without meaningfully changing anything, i.e. just redistributing Debian or Ubuntu with some logos and desktop backgrounds - there is no reason to install this on your own and no one will care. This is effectively the same thing as customized Windows installs that they ship.
Creating a custom Linux distribution. This is feasible and has already been done (System76 created Pop_OS! based on Ubuntu; Android and ChomeOS are essentially new Linux distributions built from the ground up, taking only the kernel and ignoring the existing ecosystem), but requires serious maintenance work to be any good and offer real advantages over existing distros.
Forking a kernel like Apple did. This has fairly limited purpose from the perspective of laptop and workstation OEMs that use regular off-the-shelf parts. This is usually done to provide support for custom hardware (Apple) or for proprietary software that requires deep integration with the kernel (VMware).
No one will think of option 1 as a serious competitor in the OS market, option 2 requires a ton of work and motivation, and option 3 is useless for these OEMs. Software just isn’t their business and a cheap copy offers no real advantages over shipping an existing thing.