Using Custom Fields

Hello. I was just wondering if there is a simple way to go about this: I need to have some text specific to every page on my website, and I want to display it in the same place in all pages using an element.

I can see that there is a {{custom_field.name}} tag that we can use, but I don't understand how to create it. I actually want to get to the dashboard section that appears in the bottom of the following documentation; https://docs.generatepress.com/article/page-header-template-tags/
but I don't understand how to reach it.

So I am just asking where I can find the 'Custom Fields' creation section. I assumed it's somewhere inside the page editor but couldn't find it there.