wordpress为页面主体添加“变量”（可能是标记标记）

Question

I am sorry for my inexperience in Wordpress, searching for terms like "variable" don't hone in on the specific concept I'm wanting to achieve. I also tried terms like dynamic content block.

How do I include an HTML tag which is recognized as either an included php file, or a call to a function, that will ultimately lead to an external PHP file? For example:

<p> here is paragraph 1 </p>

{{widget_name_here}} <!-- this would be replaced by a php file or include -->

<p> here is paragraph 3 </p>

-- EDIT -- I am going to Pages > New Page and in the large text area, this is where I'm entering HTML content. This is where I would like to add some type of tag like {{widget_name_here}}

In addition, there is a column to the right that says Page Attributes, and for template I am selecting Default Template.

Oh, I need to do this without any plugins due to constraints. I'm pretty sure WP has a native way to do this. Thanks