If I delete one of my tezos account in ledger live ( I already removed all the XTZ inside that account), what will happen? I want to know what happens under the surface technically. Can I reuse it or can somebody reuse it? Are they still inside my hardware wallet ledger?They still in the Tezos blockchain, right? And if I create a Tezos account in ledger live again, the previous public address that I mentioned above could reappear again?
Ledger Live, Temple, Kukai, and other wallets use HD key generation which allows for an infinite number of cryptographic secrets to be generated from a single master seed. The master seed is the 12-24 word mnemonic you generated during the setup of your Ledger Nano S/X which should be written down and safely stored.
All Tezos accounts on Ledger start with a default derivation path of 44'/1729'/0'/0' and if we add another account it will iterate through. When you add an account it will keep picking the next derivation on the path.
44'/1729'/0'/0' (Account Default) 44'/1729'/0'/1' (Account #2) 44'/1729'/0'/2' (Account #3)
When you delete an account from Ledger Live it doesn't delete the keys it removes the account from the dashboard and stops looking up balance data for it when syncing. If you remove the account and add new one Ledger will scan the path showing the "deleted account" as an option to add back to the dashboard.
TLDR; No you can't delete derivation paths only remove data. No other people can not reuse this account it is derived from your private key.
Nobody can re-use it. Your ledger device does not actually store addresses. The device has a unique seed and derives addresses and subaddress from that seed. You cannot "remove addresses" from the device without changing the seed. The address will always exist in the blockchain, but only you have the secret key. After several cycles of 0 balance, the chain will garbage collect it (ie: stop paying attention to it) and current nodes will have no record of it. Archive nodes will always have record of it.