Linux, being a kernel, has no external dependencies (except for its menuconfig, which requires ncurses). That sort of isolated build tree is very rare in almost every other software project, even something like gcc has a dozen external dependencies IIRC.
It does, including at least a shell, a C++ compiler, make, and a bunch of other UNIX/GNU tools to run the whole thing. And where do you get those programs from, and can you trust them? If you look really seriously, a scary problem emerges. (See: Trusting Trust Attack)
The Guix project recently hit a massive milestone in untangling this, by creating a package set which starts by building a 357-byte program and works its way up to the utilities you'd expect: https://guix.gnu.org/blog/2023/the-full-source-bootstrap-bui...
