How Do I Dynamically Call Data (A Page? Rich HTML?)

So I have been fumbling around metafields, metaobjects, and dynamic data. What I want to do, I can’t seem to figure out how to do.

I want to creat a single document, preferably a file honestly, that I can easily call onto a shopify product page.

I’d love for it to go right under my product description, but it seems like anything I do there is just unique to that one product. I want to be able to have this put in my theme so every single product “calls” this file into it, that way I can change the file once, and it changes that information for every product.

I have gotten it kind of working with Pages, and bellow the “buy now” button, down at the bottom kind of where I don’t want it. I’ve been able to make it load a page there with some info.

Seems clunky, also annoying to edit. It has a lot of links etc and Shopify is a bit of a mess for editing like that, everything is so close and small.