What is the relationship between Back-end and Front-end frameworks?

Question

Backend

Another noob question. All the time I thought I knew how the Back-end and Front-end work (while writing in C and Python).
The issue is that I don't know how the frameworks are related. Let's take React JS and Flask as an example. Do you need something special in order to work with them together, in conjunction? Second question. Which tasks need to be solved on the server, and which on the client side. I'm not asking for a complete list, just the gist and a couple of examples.
Thank you for your attention.