I’m interested in sending BTC from wallet to another and probably automate it through some libraries.
I have a question: if I have a seed hex or 12 words I can generate all my wallets (addresses, private keys) with BIP39 mnemonic tool.
How could I know what of external wallets (private keys) I should use to send btc using libraries assuming it usually changes?
How could I basically send BTC to some wallet having only seed hex or 12 words?
How could I know what of external wallets (private keys) I should use to send btc using libraries assuming it usually changes?
You can't without scanning the entire blockchain to find out what addresses have been used before.
How could I basically send BTC to some wallet having only seed hex or 12 words?
You can't without scanning the entire blockchain to find the UTXOs that you can spend.
In general, a mnemonic phrase or a seed is not enough to be able to send Bitcoin to someone else. You need to generate keys and scan the entire blockchain to find UTXOs that you can spend. This is why many wallets store more than just the seed phrase in their wallet files.
This Bitcoin example, posted almost 3 years ago, should succinctly answer most your questions. BIP 44 paths levels (m or M / purpose' / coin_type' / account' / change / address_index) can be tweaked as needed. Some of the adjacent examples demonstrate how BIP 32/39/44 technology can be extended to other altcoins.
For a given coin type, blockchain scanning of synthesized addresses, associated with a given sequential M, is required to identify applicable UTXOs. For a given M with associated UTXOs, there is a companion m. HD wallets applications innately perform such bookkeeping operations. HD wallets also tend to use compressed private keys, compressed public keys, and compressed addresses to minimize transaction fees.
Either use something like bx (bitcoin-explorer) to synthesize the keys of the other wallet and use the synthesized addressed to send funds, or use commercial products like Trezor, Ledger, Jaxx, Mycellium... However, HD paths and details for Ethereum and Monero will vary and will introduce seed word interoperability issues.
For example, Ledger Ethereum paths do not follow the BIP 44 Standard. Also, details for standardizing Monero key synthesis from BIP 39 seed words is non existent. This is likely to create seed word interoperability issues between existing Ledger support and future Trezor support.
Existing functionality at https://github.com/bholben/seaweed can be extended to accomplish what you desire on a old decommissioned permanently air-gapped iOS or Android device to synthesize addresses for another HD wallet.
