I just can't understand how can i use the 12 words instead of writing all the numbers related to the address/private/public keys. When i go to iancoleman.io the output is several addresses, private and public keys...but from all of this how can i get my own?
And all the private keys that come with the output, were they not supposed to be private? Can't someone just use them to transfer cryptos? This is really confusing for me.
The twelve word mnemonic phrase is a recovery phrase, also sometimes called a seed phrase or backup phrase. Knowledge of this phrase gives anyone control over all money in a typical wallet. The purpose of this phrase is to allow you to re-create your wallet if, for example, you lose, damage or destroy the device on which it is installed.
You must keep your recovery phrase secret.
From the recovery phrase a new wallet installation can generate a seed number and a master private key. From that one number, a Hierarchical Deterministic (HD) wallet can generate further private and public key pairsas needed. All of these individual private keys also need to be kept secret.
Examples
You don't need to write down all the keys, you just need to write down the mnemonic: all keys are calculated from the mnemonic. The same mnemonic in different wallets will calculate the same keys.
You will end not with one, but with several private keys (and corresponding public keys). For privacy reasons you should not use the same public key twice in transactions (people will find out those two transactions belong to the same person). That's why wallets generate new key pairs (public + private) for each transaction.
But don't worry, your wallet will do this for you. You just need to write down the mnemonic and keep it safe.
For a more technical dive on how this work, check this out.
With a BIP-compliant HD Wallet, you need not concern yourself with the addresses and privkeys yourself, everything is deterministically generated from your Seed Recovery process.
So, first off, is your account-wallet's 12-word mnemonic BIP39 compliant? Some wallet-clients like Electrum can use proprietary schemes.
Next, did you make use of a BIP39 passphrase? That is just as much part of Seed Recovery as the mnemonic.
Next, did you stick to standards-based Derivation Paths in setting up your node tree?
There are wallets using exotic derivation paths with BIP39, completely arbitrary private/public key trees.
So, it may be necessary to know which wallet-client software generated the account-wallet's seed, maybe even which version. And which client you want to restore your account-wallet to.
Also what coins / networks, script type (SegWit?)
Gap limits may come into play as well.
None of this means your assets are not kept safe on the blockchain, so long as you keep hour Seed Recovery secret, obviously never enter into any device capable of connecting to the internet.
More like, all these other factors may prevent your new wallet-client from automatically "discovering" your assets and showing a correct total balance, without your needing to give it more information.
Some wallet-clients only support a single Account / path per Seed wallet, others are multi-account.
https://github.com/bitcoin/bips/blob/master/bip-0044.mediawiki#Account_discovery
