SHA256(string) = 00000000000000000012b4d3986756787ee41599db2f83269dad8dc98d94df17
Am I trying to get the string? Is this called mining?
not exactly. what you call "string" is something called a block header. it is fixed size and consists of certain parts, some of which are fixed and some can change. for example it has a reference to its ancestor (previous block header hash) and a variable 32-bit integer called nonce that can be changed.
you keep changing the variables and compute the hash (SHA256 of SHA256) until you find a hash that when converted to a 256-bit integer is smaller than the current network's difficulty target.
when you found the correct header you can publish it as a found block.
2nd question. If I mine a block (like the last one that exists right now for example), do I get all the fees of all people that made a transaction?
yes, every block must have a first transaction called coinbase that will pay you two things: 1) the block subsidy (currently it is 12.5
BTC) and 2) the total transaction fees. the miner has to create this transaction and set its value correctly before starts mining (build merkle root hash and put it in header).
3rd question. Here's an image:
https://i.imgur.com/nX5K4Cs.pngIt says: "Miner: " and then it has a name of a company (Poolin)? What? We're talking about cryptography here, how do they know who mined it? Isn't bitcoin anonymous?
there are 2 methods that we can
guess who mined a certain block.
1) (most common) to look at the coinbase transaction's signature script and decode it as a string (text) and see if it has the name of the pool. most pools include their own name in it like Poolin then we can
assume it is mined by that pool.
2) looking at the outputs of coinbase transaction (the receiving addresses). sometimes pools have known addresses which they use to pay their miners.
note that i say "guess" because we can never be certain the name is real. for instance i can solo mine a block (although it is not possible with the current difficulty within reasonable time, but lets assume it is) and put any name inside that like Poolin. you have no way of knowing whether it was a lie or not.
read more:
https://www.coindesk.com/learn/bitcoin-101/how-bitcoin-mining-workshttps://en.bitcoin.it/wiki/Difficultyhttps://en.bitcoin.it/wiki/Targethttps://en.bitcoin.it/wiki/Proof_of_work