The best part is that you also maximize your knowledge as you learn how to build your own Arduino Board.
As a piece of advice:
If you want to have Arduino board to build a certain application that you want to make quickly so I must advise you to purchase a ready made Arduino Board.
This will be better for you as it can be easier to handle and troubleshoot so you can focus on your main project.
But if you want to build your own Arduino Board instead of buying a ready made one as a way to start learning Arduino programming from scratch then I encourage you to start building it step-by-step.
How to build Arduino on a breadboard
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