How do I create a web application using Core Python without a framework?

While there is a discussion in the comments about the utility of frameworks, I am trying to answer the question at its face value.

WSGI

Python frameworks like Flask and Django both at the end are WSGI applications. WSGI (Web Service Gateway Interface) is a PEP specification which defines how the server and client must communicate. If I were to start from scratch, I would probably start with learning about WSGI and even try implementing a little ping-pong server with it. The read the docs page here https://wsgi.readthedocs.io/en/latest/learn.html lists a number of pages to learn about it.

Werkzeug

Once the WSGI specs are understood then one can attempt building a simple library that will wrap the core concepts into reusable functions and modules for easily writing an application. Here Werkzeug can be a good guide to make one understand the different aspects. https://www.palletsprojects.com/p/werkzeug/

Your own application

Based on the understanding of the WSGI spec and the Werkzueg library you can go on to write your applications either from scratch, or write a library like werkzueg yourself and then use it to write an application.

Finally reimplement the same app in Flask or Django to see what frameworks offer.

If it's something small for internal use you can use https://docs.python.org/3/library/http.server.html