which is usually/often an external command, it doesn't know about shell aliases, or shell functions for that matter.
type python would recognize the alias, and type -a can be used to show all matches of the given name. It doesn't track where the alias points to, though, just shows the pathname that would be used if there was no alias:
$ alias python=/usr/bin/python3
$ type -a python
python is aliased to '/usr/bin/python3'
python is /usr/bin/python