Based on what I saw 1-2 years ago last time I looked at it, most distributions to customize and don't use the defaults straight up. From memory, so someone correct me if I'm wrong:
- RHEL/SLES - Lots of patches to kernels
- Arch - Closer to just using defaults, some config choices and downstream adjustments (so the opposite of CachyOS almost, which is why we have CachyOS in the first place)
- Ubuntu - Probably the most patched distribution compared to upstream components, also includes a lot of Canonical-specific stuff on top of that.
- Fedora - Has some bleeding edge bits and bobs
- Debian - Bit more conservative than Ubuntu, but still has patches for stability, security and backports.
In my experience, distributions changing the defaults and customizations seems to be the norm rather than the exception.