[Python] How does creating a Python venv with `--system-site-packages` work under the hood?

I'm trying to understand how creating a Python venv with access to the system site-packages work. The documentation doesn't go much into details, so I've just checked the diff between two venvs with and without the --system-site-packages option. It seems that the difference is just in pyvenv.cfg:

@@ -1,5 +1,5 @@
home = /usr/bin
-include-system-site-packages = true
+include-system-site-packages = false
version = 3.12.8
executable = /usr/bin/python3.12
-command = /usr/bin/python -m venv --system-site-packages /tmp/venv1
+command = /usr/bin/python -m venv /tmp/venv2


How does this make the system packages accessible in the virtual environment?

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