That demo GIF is just showing a toy example. To see what it's like to work with aider on more complex changes you can check out the examples page [0].
The demo GIF was just intended to convey the general workflow that aider provides: you ask for some changes and aider shares your existing code base with the LLM, collects back the suggested code edits, applies them to your code and git commits with a sensible commit message.
This workflow is generally a big improvement over manually cutting and pasting bits of code back and forth between the ChatGPT UI and your IDE.
Beyond just sending the code that needs to be edited, aider also sends GPT a "repository map" [1] that gives it the overall context of your codebase. This makes aider more effective when working in larger code bases.