But there are some commonality between all. Like you will be understanding and learning about your product in beginning of your career by doing small projects. In your mid career you will be responsible to perform or lead small or mid size projects with minimal supervision. and once you become senior in you career. You have to learn and perform by your self and guide junior level engineer also.
If you chose R&D: Than I would say your day would be like developing concept and doing analytical studies to validate you concept. And build the prototype to give the actual shape to your idea. And working in test lab to run various kind of tests to see how your product react to those conditions. You will working with Vendors/supplier to source your parts and explaining them the specifications to bridge any kind of communication gap. Participation in design review meeting to share your results or hurdles. And if everything will be delivered on time than of course after work Beer and food on company expenses.