I kinda have to agree with @jamestwebber here. They’re handled in the sense that people are building their code, but it doesn’t seem very elegant, especially for large monorepos. I’ve already seen at least one example of a custom system whereby a tool (itself part of the monorepo) dynamically generates pyproject.toml files based on importing a module and checking for an attribute - implementing something like a precursor to PEP 723 but for building instead of running. And I thought part of the goal was to standardize requirements files.
Coincidentally, you posted just as I was submitting my outline and intent to produce “more details”, and in the few hours since my previous post ITT, I spent most of my thinking time on the point (1) described there which comes primarily out of your feedback here. Your insight is greatly appreciated.