Does that mean the swank thread still gets all the inputs of the live process and you can debug in isolation there and then push changes to the main thread? Sorry if I’m stupid (no need to reply), I need to read into this more.
You don't need to "push changes to the main thread", you can mutate the state of the whole process during debugging, and in Lisp that includes compiling and loading new code that gets used immediately by whatever application you're trying to debug.
It's probably worth mentioning that you usually don't want to make changes in a live production environment, and even when you do, you probably want to capture those changes in a file as well, so that you have a record of what you did. It's possible to make arbitrary changes to a live, running system in Lisp with no other record of what changes you made, but it's not usually a good idea to do that.