> how to define a python version for a specific project (it's defined in both .python-version and pyproject.toml)

pyproject.toml is about allowing other developers, and end users, to use your code. When you share your code by packaging it for PyPI, a build backend (uv is not one, but they seem to be working on providing one - see https://github.com/astral-sh/uv/issues/3957 ) creates a distributable package, and pyproject.toml specifies what environment the user needs to have set up (dependencies and python version). It has nothing to do with uv in itself, and is an interoperable Python ecosystem standard. A range of versions is specified here, because other people should be able to use your code on multiple Python versions.

The .python-version file is used to tell uv specifically (i.e. nobody else) specifically (i.e., exact version) what to do when setting up your development environment.

(It's perfectly possible, of course, to just use an already-set-up environment.)