Get Python Path to Executable within the QGIS Python Console

python_path.py

# using sys.path
# see also https://github.com/qgis/QGIS/issues/45646#issuecomment-950339443

import os
import sys
import subprocess

def find_python():

    if sys.platform != "win32":
        return sys.executable

    for path in sys.path:
        assumed_path = os.path.join(path, "python.exe")
        if os.path.isfile(assumed_path):
            return assumed_path

    raise Exception("Python executable not found")

python_exe = find_python()
subprocess.check_call([python_exe, "-m", "pip", "install", "geojson"])

python_path_qgs_libpath.py

# using qgis.core.QgsApplication.libraryPaths()
# not sure that this is the best way

import os
from sys import platform

import qgis.core as qc

def get_python_path():

    # QgsApplication > QApplication > QGuiApplication > QCoreApplication
    # https://doc.qt.io/qt-5/qcoreapplication.html#libraryPaths
    # https://doc.qt.io/qt-5/qt-conf.html
    # https://github.com/qgis/QGIS/blob/f401ec2c221f2711e8042e750c0560debb48e34e/src/app/main.cpp#L1086

    for path in qc.QgsApplication.libraryPaths():
        
        assumed_path = os.path.join(path, "python3") # linux
        if platform == "win32":
            assumed_path = os.path.join(path, "python.exe")
        
        if os.path.isfile(assumed_path):
            return assumed_path

    return None

print(get_python_path())