Best online platform to learn about Raspberry pi and Arduino board

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I have a project in my mind from a long time which is related to Raspberry pi and Arduino board but unfortunately I don't know much about these portable boards to work on. I would like to know some websites where I could learn about Raspberry pi and Arduino board. I would like to learn how to code in these boards and what are some practical projects made with these boards so far. But every time when I browse the Web to gain some knowledge about these topics I end up with only basic information about their architecture and arrangements. I would like to explore the practical projects made using these boards.

Also specify if there is any other similar kind of board used for innovative projects.