I wrote some words (with an example!) about how to write custom async on-demand importers in Unity: https://blog.s-schoener.com/2024-11-22-unity-async-imports/
Unity Async On-Demand Imports

Hey! You can find me on Mastodon, Bluesky, or Twitter! For a while now, Unity has had a neat little feature: async, on-demand imports. It’s not widely used or implemented, but it exists. This blog post is going to point you in the general direction of how to implement such...

@sschoener oh wow, that is pretty cool! I’m wondering how many of “yeah this is actually possible” workflows are possible, just Unity does not clearly tell people about them
@aras My favourite undocumented workflow are AsmRefs into immutable packages :)