Isn't there a HTML file that applies to all post files?

I wanted to implement this code:

{{ page.title }} {{ content }}

But not sure where to implement it since I created a new _layouts file to use layout: default to use Mathjax. Where’s the HTML file that includes the layout: post modes and etc.

ps: sorry about my terminology if it’s not totally accurate.

Without further information (are you using a theme) difficult to answer. One possible solution for your: Search through all .html files of your source for “{{ content” (with or w/o space after “{{”. to see how it’s done so far. Searching thrower .md files for this might bring additional ideas. [A text editor like BBEdit allows easily searching through entire project, your preferred editor/platform probably has similar capability.]