uv python

gistfile1.txt

uv init (defaults to --app)
gives:
├── .git
├── .gitignore
├── .python-version
├── main.py
├── pyproject.toml
└── README.md

uv add <package>

And then you can check which package installed/comes with which packages/deps:
uv tree
Resolved 6 packages in 3ms
test-aws-nova-act v0.1.0
└── requests v2.32.5
    ├── certifi v2025.8.3
    ├── charset-normalizer v3.4.3
    ├── idna v3.10
    └── urllib3 v2.5.0

If you change the version in .python-version and it had the version from initial "uv init" run sourced from system then it will throw error with the context of current dir:
uv tree         
Using CPython 3.12.11
error: The Python request from `.python-version` resolved to Python 3.12.11, which is incompatible with the project's Python requirement: `>=3.13` (from `project.requires-python`)
Use `uv python pin` to update the `.python-version` file to a compatible version

If if it a fresh project with basic uv related files but not with .venv then you can simple run:
uv sync

what if there is an existing project that uses plain requirements.txt and might also have existing venv config, how to migrate from it?
uv init

But how to get those packages from requirments.txt?
uv add -r requirements.txt
> this will auto update pyproject.toml and uv.lock

uv python install <python-version>
uv pip uninstall <package>
uv pip install <package>

uv pip freeze > requirements.txt
uv pip install --refresh -r requirements.txt