Tectonic is a cool project, but hasn't seen any significant changes in a few years---and likely won't anytime soon. It seems we maintainers don't have the time and motivation to put serious work into Tectonic.
I haven't looked at the code in years (and thus may be wrong), but here's a quick overview:
Tectonic's code consists of thin bindings to /harfbuzz/graphite/etc and a vendored XeTeX (in C, with some tweaks to make the build easier), driven by Rust that tries to keep the TeX environment predictable and sane. A few components have been fully ported to Rust (bibtex, spx2html), but the project is very unfinished.
I've looked into the dark corners of TeX when I worked on Tectonic, and it is not pretty. TeX relies on a stack of evil hacks and esoteric behavior that is very hard to replicate, and very difficult to expose in an ergonomic way. This is true of the core system, and of many packages on CTAN.
A quick example: code highlighting does not work in Tectonic. The canonical solution is https://ctan.org/pkg/minted, which spawns a python process to style your code. Reproducibility is one of Tectonic's selling points, so we cannot replicate this behavior.
With https://typst.app/ as good as it is, there's little motivation to modernize TeX---especially considering the effort required. Typst _is_ modern TeX, and I'd rather spend my time there.