It's not "some sort of technical distinction". Package managers are for keeping track of which pieces of code you need in your project's environment. Build systems are for... building the code, so that it can actually be used in an environment.
Usually, you can directly make a pre-built wheel, and then an installer like Pip or uv can just unpack that into the environment. If it needs to be build on the user's machine, then you offer an sdist, which specifies its build backend. The installer will act as a build frontend, by downloading and setting up the specified backend and asking it to make a wheel from the sdist, then installing the wheel.
Poetry's build backend (`poetry.masonry`) doesn't build your external dependencies unless a) you obtain an sdist and b) the sdist says to use that backend. And in these cases, it doesn't matter what tools you're using. Your installer (which could be Pip, which is not a package manager in any meaningful sense) can work with `poetry.masonry` just fine.
If you can give a much more specific, simple, reproducible example of a problem you encountered with external dependencies and uv, I'll be happy to try to help.
> Package managers are for keeping track of which pieces of code you need in your project's environment. Build systems are for... building the code, so that it can actually be used in an environment.
This probably means something to the developers of the package managers and build systems, but to me, as a Python developer who wants to be able to publish a pure Python CLI program to PyPI, it seems like a distinction without a difference.
>Seems that if I want my package to be installed, I need to pick a build system, regardless of if I am using any native code.
If you want to distribute it to be installable by others, yes. Except that at least for now, installers will assume Setuptools by default if you don't mention anything in your `pyproject.toml`.
>but to me, as a Python developer who wants to be able to publish a pure Python CLI program to PyPI,
If you're making pure Python projects, the actual build process is trivial and every build system will do just fine. But again, the build system you choose builds your code, not your external dependencies. Whatever you put in `pyproject.toml` here has nothing to do with the packages that you install. It has to do with other people installing your package (and you taking steps to make that easier). So "external dependencies don't build the same as on Poetry" makes no sense in this context. If you need to build external dependencies (i.e. they don't come pre-built for your system), they will automatically be built with the build system that they choose.
>it seems like a distinction without a difference.
Let me try again: When you use a package manager, it's so that you can keep track of which code from other people you're using. When you choose a build system and mention it in `pyproject.toml`, it's so that other people can use your code. (For your pure Python project, you will normally run the build system locally - https://pradyunsg.me/blog/2022/12/31/wheels-are-faster-pure-... . But the overall packaging ecosystem is designed so that you can push part of that work onto the end user, when it makes sense to do so.)
Usually, you can directly make a pre-built wheel, and then an installer like Pip or uv can just unpack that into the environment. If it needs to be build on the user's machine, then you offer an sdist, which specifies its build backend. The installer will act as a build frontend, by downloading and setting up the specified backend and asking it to make a wheel from the sdist, then installing the wheel.
Poetry's build backend (`poetry.masonry`) doesn't build your external dependencies unless a) you obtain an sdist and b) the sdist says to use that backend. And in these cases, it doesn't matter what tools you're using. Your installer (which could be Pip, which is not a package manager in any meaningful sense) can work with `poetry.masonry` just fine.
If you can give a much more specific, simple, reproducible example of a problem you encountered with external dependencies and uv, I'll be happy to try to help.