These are all good questions. They do go beyond the scope of unit 4 because the focus of unit 4 is designing the app and not programming it. I know students would like to go beyond that and start programming and of course, you as the teacher can make that choice to let them do that, but there isn’t explicit instructions provided in unit 4 to do that.
The nitty gritty details of using app lab can be found in the high school CSP Curriculum in units 3 & 4.
There was also a module created during the pandemic called “Event-Driven Programming in App Lab.”
Lesson 7 of that module is all about Inputs and Strings and if I had an advanced student who wanted to learn more, I would probably send them the link to that lesson and have them learn from that. Once they mastered that, they may be ready to move on to figuring out how to get the value of a radio button. That will involved some conditionals and event listeners to determine which radio button was pressed.
I don’t have a working prototype of that to share, but it would work somewhat like the example found in this thread where they used a drop down menu and changed the screen color based on that. It may even be easier to use a drop down than a radio depending on what they are doing.
So, again, if the kids are ready for that and you have the time, hopefully here are a few resources, but it goes beyond what most of my students are ready for in unit 4, but may provide some challenging work for students who are earlier finishers!
Mike