Oh also OP i forgot to say that whilst there isn't much engineering in product design A level it does compliment those other subjects well and shows you have a more creative side
a lot of detail tbh (you keep a written portfolio throughout the project), there is 4 main stages i can think of:
-initial ideas, this takes a few months as you generate an idea based off a problem usually, for an example you identify an area that doesnt work in a product and would redesign that product for a client. you would then plan this out with sketches
-Modelling, a large stage, physical modelling using paper, board etc or sometimes MDF etc
-Manufacture, the making stages of the product, writing how you make it as you go along
-Testing and evaluation, testing your product against the initial criteria of what you wanted it to achieve and also compiling client feedback
you use 2D design which is a CAD programme if you are manufacturing your product using a CNC machine such as a laser cutter or the CNC router, at the end you also make a technical drawing on CAD which is designed to be read by a manufacturer so that theoretically you could make it in industry. Sometimes a model is also made in solid works (a 3D CAD programme) of how the product looks at the end
as for the science bit it depends on the product really, mine was focusing heavily on function this year so i did a lot of research on properties of materials etc but that would be of lesser important if you are focusing on aesthetics
hope that helped