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How are Bitcoin blocks structured?

Bitcoin blockchain is made of individual blocks. I am wondering how do these blocks actually look like. Which different data types are part of them? What is each part of the block used for, what is its meaning and what are the possible values that can those values take?

I would also like to, assuming I am looking at a block’s HEX blob, be able to decode it/determine various parts of it.

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How to create a valid Bitcoin transaction?

If I want to create a Bitcoin transaction, which are the parts I should add to it? What is the meaning of each part and what are any special considerations that I should think of?

I would also like to, assuming I am looking at a transaction’s HEX blob, be able to decode it/determine various parts of it.

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If transaction hashes only are stored in blocks, then where is OP_RETURN data stored?

An answer to a previous question and the whitepaper both agree that blocks don’t store full transactions, but rather, transaction hashes.

So then, what if I add data in OP_RETURN, say, the text “Hello, world!” Isn’t that a transaction output, or at least contained in an output, and therefore part of a transaction?

If not, or if so and only transaction hashes, not transactions themselves persist, then how does the OP_RETURN data stay on the blockchain?

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How does IP2IP work?

I heard a lot of discussions mentioning IP2IP at the recent CoinGeek conference in Seoul. I really like the idea of users exchanging transactions peer-to-peer, but I am wondering how IP2IP works on a technical level?

Does it require a daemon/service running on the recipient’s computer that listens for incoming transactions or is there some other approach?

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Which transactions are available from "getrawtransaction" RPC call when "-txindex" is disabled?

I noticed that even with -txindex disabled getrawtransaction RPC call still returns transactions. I assume that without the full index it will still return transactions that node software needs for other reasons (wallet transactions, UTXOs, …). But what are those cases? Which types of transactions are available even without the full transaction index? And what happens if I request a transaction that is not available?

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