First, look at the bold part of my EDIT at the bottom! I think the original way I posed the question is confusing people.
I've followed the instructions here to switch my Alt and Ctrl keys. Now when in a terminal I want to press the physical Ctrl+C to cancel. But the new mapping means I physically press Alt+C.
and can't find that shortcut there.
I am on Ubuntu 20
I realised I might be confusing the problem by telling you I originally switched my Alt and Ctrl keys. You can mostly forget about that if it makes it easier. Think of the question as: "Instead of pressing ctrl+key in the terminal for things like process interruption (ctrl+C) or reverse search (ctrl+R) I want to press alt+key. How can I set that up without actually changing the system-wide key bindings in general?"
Finally, if it helps to understand my motivation for wanting to do this: right now, under my key mapping I have to physically press Shift+Alt+C to copy from the terminal, whereas in every other app that I use I just press Alt+C. After months I still can't get the hang of mentally adjusting. I end up pressing Shift in my other apps, or cancelling things in my terminal by forgetting to press Shift in the terminal. It's driving me insane!
Another point on my motivation. I'm coming from being a mac user, where in the terminal you have to press physical Ctrl+C to cancel, and physical Cmd+C to copy. On a standard non-mac keyboard, the Cmd is in the same place as the Alt. That's why I've done my key mapping this way.