§ How pip decides
1
pip install flask — ask PyPI what boxes exist for the chosen version.
2
Is there a wheel whose tags fit this machine (Python version, ABI, OS/CPU)?
├─ yes → download it, read METADATA, unzip. No code runs to install.
└─ no → download the sdist and build it — flask 0.1's world, where every install ran setup.py.
3
Read the dependency constraints and recurse — then resolve them all simultaneously (the backtracking search of Sheet No. 2, §3).