If you take a look at this site,
while the container is “container-fluid”, every element appears in the center of the page, such as the “container” class.
Not sure what you are specifically referring to on that page?
But container-fluid is a full-width container. As such child elements can only be horizontally centered inside it if they are not block-level elements or have a width limit imposed. Otherwise, they will just be the same width as the container.
Horizontal centering in Bootstrap is usually done using an auto margin mx-auto or flexbox, or if possible content can also be centered using a simple text-align.
If you want vertical centering of block-level elements you have to use flexbox (or some old-school use of positioning).
<div class="container-fluid min-vh-100 w-100 d-flex flex-column justify-content-center align-items-center">
<header class="header w-75 p-3">
<h1>Welcome to the page</h1>
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</header>
</div>