I use obsidian for just text stuff I need to remember later. If I need to do math or need a diagram temporarily I prefer pencil and paper. If I want to save diagrams for later I have a drawing tablet that I use like a whiteboard so that I can save and use layers. If you don’t have a drawing tablet Autodesk sketchbook is a decent substitute that can be used on your phone.
I use obsidian for just text stuff I need to remember later. If I need to do math or need a diagram temporarily I prefer pencil and paper. If I want to save diagrams for later I have a drawing tablet that I use like a whiteboard so that I can save and use layers. If you don’t have a drawing tablet Autodesk sketchbook is a decent substitute that can be used on your phone.