I also have a nice notebook with dotted grid paper that I use when I’m feeling more precise with my thoughts and want to capture them more permanently, or just more elegantly.
Other than that I’ll use TextEdit or Notes.app on the iPhone, but often will transcribe that stuff to a card so I have everything together. Some stuff I’ll leave digital if I know I’ll need to keyword search for it later. But, generally, physical cards are pretty easy for remembering where things are.