Bottles are cached in a content addressed store by the sha256 from the registry install manifest. All install requests are resolved live against the registry, so there is no local metadata cache to invalidate.
There is no mirror selection, all bottles are downloaded directly from GHCR's CDN.
The speedup comes mostly from one shot resolution, intelligent download scheduling and parallelization of install and postinstall work.
Packages are installed to /opt/glustore so they don't affect Homebrew installed packages.