How do you create custom Twig functions or filters?

Twig supports template inheritance with a strong system wherein one template can extend another. This allows for reusable, modular code. A base template, generally containing shared elements of headers, footers, and layout structure, would define sections using the block tags that child templates can override. Child templates then use the `{% extends ‘base.html.twig’ %}` syntax to inherit the base template’s structure and selectively override blocks as needed. This follows the DRY principle, and this kind of approach provides an efficient way to manage consistent layouts in big projects: common elements are kept in one place, and only what’s necessary is overwritten in child templates.