Typing export every time is tedious, so projects keep their settings in a file named .env:

API_KEY=abc123
APP_MODE=development

A small package called python-dotenv loads that file into the environment when your program starts:

from dotenv import load_dotenv
import os

load_dotenv()                       # read the .env file
key = os.environ.get("API_KEY")
print(key)

One critical rule: never share your .env file. Add it to .gitignore so it's never uploaded to GitHub. Share an .env.example with blank values instead, so others know which settings they need to fill in.