By default, Sage will try to use system's python3 to set up a virtual environment, a.k.a. venv rather than building a Python 3 installation from scratch.
Sage will accept versions 3.9.x to 3.12.x.
You can also use --with-python=/path/to/python3_binary to tell Sage to use /path/to/python3_binary to set up the venv. Note that setting up the venv requires a number of Python modules to be available within the Python in question. Currently, as of Sage 10.3, these modules are as follows: sqlite3, ctypes, math, hashlib, socket, zlib, ssl, ensurepip.
Use the configure option --without-system-python3 if you want Python 3 built from scratch.