Short answer: at the core of your second brain, especially in those places where you manage “personal projects.”
Longer answer:
The PARA method integrates tasks and notes in a way that favors actionability. Therefore, it is mainly used in one's main PKM app (Evernote, Notion, Obsidian, etc.) or Personal Knowledge Companion (PKC).
However, since PARA is nothing more than organizing by actionability, you can theoretically implement it in any tool involving some action and/or notes.
Here are some ideas:
Task Artifact Manager
Internet Browser
Email client
File System
Read-Later Apps
Forte himself uses the method only in his PKC (Evernote), his task artifact manager (Things 3), and his file system on macOS. These sit at the core of his system, and he mentioned before that everything else outside of this core probably does not need them. However, to qualify that a bit, I would say that you can productively implement it in the core of your second brain (whatever that may be).
Framed differently, it makes sense to implement PARA in all places where one engages in "personal project management." That is, where one wants to manage containers of work that involve tasks and/or notes. If, for you, that involves something like YouTube, Spotify, your podcast app, etc., these tools probably also benefit from establishing PARA there. But all other tools don’t.
Personally, I tried it out in many places but settled for PKC, task manager, file system, and a varied version in my Arc Browser. I found that it only added additional work everywhere else without much benefit. I worked with this setup for many years productively. However, at some point, I moved personal project management out of my task manager (Todoist) and over to my PKC. And that meant I also got rid of PARA in my Todoist. Since Forte does PPM in Things and Evernote, having PARA in both places makes sense. However, if you use some other task management system like GTD, I recommend sticking with that and only managing projects inside your PKC. Since with my current PKC (Obsidian), there is no longer any difference between PCK and file system (notes are files in the file system and vice versa), I can say that I used it in one place at the end.
I have since moved on from PARA (and personal project management). Instead, I have now adapted my very own method called PEAKER.
Knowledge Builders, a Fractal Productivity spin-off, extends PKM to Personal Knowledge Building (PKB). And PEAKER plays an important role in that.