Anki's UI is primarily written in Python/PyQt. A number of screens, such as the review screen and editor, also make use of TypeScript and Svelte. To write add-ons, you will need some basic programming experience, and some familiarity with Python. The Python tutorial is a good place to start.
Add-ons in Anki are implemented as Python modules, which Anki loads at startup. They can register themselves to be notified when certain actions take place (eg, a hook that runs when the browse screen is loaded), and can make changes to the UI (eg adding a new menu item) when those actions take place.
There is a brief overview of Anki's architecture available.
While it is possible to develop Anki add-ons with just a plain text editor, you can make your life much easier by using a proper code editor/IDE. Please see the IDE & Type Hints section below for more information.
At the time of writing, Anki depends on Qt 5.14 or 5.15. You may find Qt's documentation useful.