I have sftp access to the root of my entire website linux volume, which contains all of the folders of dependencies. I see the Cloudron installation and its dependencies. The entire file structure of my website server is open to me. There is no other volume adjacent to this Linode volume. Nothing can exist outside of this root in my server volume.
Is still factually wrong, since you seem not to know or understand that each container has their own filesystem in their isolated container environment.
So even if you have root access to your server, you will not find /app/code in the servers filesystem.
Since there are isolated and it's inside the container and read-only.
And yes you can still access each container like he even explained to you how you can access the filesystem:
To navigate the filesystem of the app, use the web terminal. Usually, the package keeps the code in /app/code by convention. Note that code is readonly in Cloudron, you cannot change it.
What do you wish to achieve here?
There is a big documentation for Cloudron: https://docs.cloudron.io/ which explains almost everything.
Adding to that the Forum has many resources which already answer almost any of your questions.
And yet it seems to me, you simple choose to ignore all the provided resources for self education and demand support on a level which is unreasonable for a forum request.
The community behind Cloudron and FOSS in general is very forgiving when you show the effort, and when you reach out for help, they almost always respond. (not always in the nicest way, since they maybe seen this question for 50x time, but they still respond to help out.)
So I recommend to you, show some more effort or at least write your request in a more descriptive way.
What is it that you are trying to achieve
What did you do to get to your current state
Provide a step by step level so people can understand what you do - maybe use a service like https://asciinema.org/
Since I preached about resources and self education - here is some material: