Where does tectonic put it’s support files when it has downloaded them?
On Linux, the files are stored in
~/.cache/Tectonic. On macOS, it should be
~/Library/Caches/Tectonic. Or, less concretely but more to the point for developers, Tectonic uses the user cache directory chosen by Rust’s app_dirs crate.
Within that directory, there are currently four subdirectories:
urls/contains saved information mapping from bundle URLs to bundle SHA256 fingerprints
manifests/contains saved information mapping from bundle SHA256 fingerprints to the SHA256 digests of individual files within each bundle, or negative entries indicating that the bundle does not contain files of a given name.
files/contains actual file contents, addressed by their SHA256 digests, stored using a Git-like two-level directory hierarchy.
formats/contains “format” files that store the initial state of the TeX engine when processing of a job begins. These are stored in the per-user cache, not in the bundles, because the contents of the file depend sensitively on the internal data structures of the engine.