Why you should prototype without assets
When creating a game it’s often better to create a prototype first without any assets. This allows for much faster development and test out new concepts. If working in a team this means being able to develop while the art isn’t done yet. Using some placeholders is fine of course. Often people take too much time and effort to search or create art for their game while all those resources could be used to develop the game mechanics.
Using the basic objects in Unity you can already create a working game. To focus on one task at a time is fastest way to get results. Whether you’re learning or creating things.