Mise installs itself as a static binary actually (but it's of course packaged in many registries), and while there are some third party registries it delegates to for some packages (aqua, asfd), most stuff I have installed is either built-in, or from PyPI, npm or GitHub, i.e. directly published by the upstream maintainers. More info: https://mise.jdx.dev/dev-tools/backends/
There's always the chicken/egg problem of which dependency manager to install first, though. AFAIK there's no "trusted" installed for Homebrew on macOS though I might be wrong.