CSS reaches a page in three ways: an external file linked with <link rel="stylesheet">, an internal <style> element, or an inline style attribute. Below, a data: URL stands in for a file such as /css/site.css.
<!doctype html>
<link rel="stylesheet" href="data:text/css,p{color:%232e7d32;font:17px system-ui}">
<style> p { color: #1565c0; } </style>
<p>External font; the later internal rule makes it blue.</p>
<p style="color: #ef6c00">The inline style beats both.</p>
Prefer external files: the browser downloads them once and caches them for every page. Reserve <style> for small critical CSS and inline styles for values set by scripts; inline styles cannot use selectors or media queries, and only !important overrides them.