Linux is an open source computer operating system created by Linus Thorvalds created in 1991. His Linux kernel has been used in several operating system projects usually called distributions. Famous distributions are Slackware, Debian and CentOS.
There is more to linux then Ubuntu or CentOS; here is a selection of for me interesting linux distributions.
articles on musl-libc
Enable zRAM for a more efficient swap system
zfs zstd compression
btrfs zstd compression
alternative directory structure
linux on 8bit and 16bit architectures
uzi, unix-like for the z80
elks, a 16bit no-mmu linux-like
different os setup
run linux on iphone/ipad at native speed https://worthdoingbadly.com/hv/