Open settings and search for Python Default Interpreter Path.

In your case, you wish to set it to /usr/bin/python3.6. Though I would suggest using a virtual environment.
{
"python.defaultInterpreterPath": "/usr/bin/python3.6"
}
If you are using virtual environment, to ensure that the terminal is using the associated python environment, check the second option Python > Terminal: Activate Environment:

In settings.json it is called "python.terminal.activateEnvironment": true.
One reason your change did not persist, was because in your screenshot, you opened your workspace's settings.json.
To make it default, you should edit your user's settings.json instead.
This you can access with Ctrl+Shift+P, Preferences: Open Settings (JSON).
Detailed instructions can be found in the documentation "Manually specify an interpreter", including using environment variables as the interpreter's path.
Also, "python.pythonPath" has been deprecated:
2021.6.0 (16 June 2021)
5. Added python.defaultInterpreterPath setting at workspace level when in pythonDeprecatePythonPath experiment. (#16485)
8. Show python.pythonPath deprecation prompt when in pythonDeprecatePythonPath experiment. (#16485)
2020.7.0 (16 July 2020)
9. Prompt users that we have deleted pythonPath from their workspace settings when in Deprecate PythonPath experiment. (#12533)
2020.5.0 (12 May 2020)
6. Do a one-off transfer of existing values for python.pythonPath setting to new Interpreter storage if in DeprecatePythonPath experiment. (#11052)
8. Added prompt asking users to delete python.pythonPath key from their workspace settings when in Deprecate PythonPath experiment. (#11108)
12. Rename string ${config:python.pythonPath} which is used in launch.json to refer to interpreter path set in settings, to ${config:python.interpreterPath}. (#11446)
2020.4.0 (20 April 2020)
13. Added a user setting python.defaultInterpreterPath to set up the default interpreter path when in Deprecate PythonPath experiment. (#11021)