The musl C library is an alternative to the equivalent GNU library (glibc) and is used by some (but few) distributions, which are generally simpler, bespoke, light in resources and with much, much less polish than GNU/Linux distributions .
Which begs the question, of course, of what to call these distributions: musl/Linux? Although many of those use busybox as the base of the userland, others use GNU coreutils, so there’s some GNU in there as well.
However, there seems to be a widespread desire of correctness and of compatibility with the POSIX standards than to be compatible with the de-facto standards, whether GNU or BSD.
It’s the Third Way!
The best-known natively musl distro is Alpine Linux, targeted for servers and very popular in containers and lightweight systems, and then ther’s Adélie Linux, for desktops, and PostMarketOS, for mobile. Other distributions (usually the ones that can be configured from scratch, like Void and Gentoo) support being built both with GNU glibc or musl)
It is usually to the MUSL library that projects that want to depart from GNU or the GPL license gravitate, like toybox (a busybox alternative that has already replaced it as Android/AOSP’s base shell ), or the s6-init, one of the few alternatives to systemd that in fact has something more to offer than nostalgia and a return to the past.