By default it prefers packages at micropython.org-s index. If the package or the required version is not found there, then it turns to PyPI.
The same tool is behind the MicroPython/CircuitPython package manager of the just-released Thonny 3.3.10.
You can give a requirements.txt to minipip and if a package has requirements, they are taken care of. It’s just that version resolution for upip-compatible packages is not as advanced as in the newest pip versions. If the package has pip-compatible assets, then pip takes care of everything.
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