Customizing Item / Single Post Template

Thank you! Can I do this from inside Wordpress? Or do I have to go through my host to access the theme root folder?

In Wordpress, I only see ways to edit existing theme files:

I did look into creating a child theme, but I had much the same problem. All the guides I can find start with steps like “create a child theme folder” with no details on where I’m able to do that or how:

Worpress keeps going more and more WSIWYG-focused, so it’s entirely unclear how I can even access the directories to begin with.

I’m sure it will make much more sense once I can actually play around with the code itself, but I don’t have the basic context for how to get that far, which I’m sure is common-sense to developers :')