Page.find() vs pages.find()

I am using the code in the template files (.html.twig): page.find (), e.g.
{% set page = page.find ('/_widgets/_about') %}

Everything seemed fine, but I noticed on the 404 error page that the page object is null.

So I changed the code from page.find() to pages.find() and everything started working on both the normal pages and the 404 error page.

So what's the difference between page.find() and pages.find()?

In the documentation, there are both versions in the sample codes.

Can you provide the documentation link about page or pages ? i just curious ..