Several companies (including mine, Toradex) have setup board farms to deal with Kernel regressions and bugs during the `-rc` window, ie, the kernel that is going to be released is heavily tested.

https://kernelci.org/ is a big one, Linaro has theirs https://lkft.linaro.org/, Intel has multiple farms, Collabora works relentlessly with GitLab integration https://www.collabora.com/news-and-blog/news-and-events/patc......

The kernel is very much tested before a release with gigantic automated labs on real hardware.