Venvs will actually contain a symlink to a python binary in its venv/bin directory. It doesn't copy a Python executable or download one from the internet as far as I can tell.

Still, this means you can install multiple system Python versions and your venv will always contain an executable file of the version the venv was last installed/updated with.

    ╭─android@localhost ~/tmp/py
    ╰─$ python3 -m venv testvenv
    ╭─android@localhost ~/tmp/py
    ╰─$ file testvenv/bin/python3
    testvenv/bin/python3: symbolic link to /data/data/com.termux/files/usr/bin/python3

Venv creates a /bin in it's environment dir that symlinks the Python version used to initialize it or as specified. That's it's entire basic model of operation and takes a few seconds to verify.