How do I get which radio button is checked from a groupbox?

You should take some look at the CheckedChanged event to register the corresponding event handler and store the Checked radio button state in some variable. However, I would like to use LINQ here just because you have just some RadioButtons which makes the cost of looping acceptable:

var checkedRadio = new []{groupBox1, groupBox2}
                   .SelectMany(g=>g.Controls.OfType<RadioButton>()
                                            .Where(r=>r.Checked))
// Print name
foreach(var c in checkedRadio)
   System.Diagnostics.Debug.Print(c.Name);

You can find all checked RadioButtons like

var buttons = this.Controls.OfType<RadioButton>()
                           .FirstOrDefault(n => n.Checked);

Also take a look at CheckedChanged event.

Occurs when the value of the Checked property changes.

groupbox1.Controls.OfType<RadioButton>().FirstOrDefault(r => r.Checked).Name

this will get the name of checked radio button. If you want to use it later, you might store name of that by storing into variable.

Cheers🍻