A Bitcoin wallet does not contain digital coins as files. It manages keys and transaction information. Some wallets can sign transactions; others are watch-only and can follow activity without possessing the private keys required to spend. Wallet documentation.
Private keys and signatures
A private key can produce a digital signature. A corresponding public key lets other participants verify it without learning the private key. Bitcoin uses signatures within spending rules; the details depend on the output type. Transaction and script documentation.
“Whoever has the key has the coins” is a useful warning, but an incomplete technical description. An output can require multiple signatures or additional conditions. An address is a way to encode a payment destination, not universally a raw public key.
The recovery phrase
Many wallets derive keys from a seed and present a recovery phrase as a backup. Formats and optional passphrases vary. Someone who obtains the necessary recovery material may be able to recreate the signing keys. Deterministic wallets.
A backup must match the wallet’s recovery method. A phrase without a required passphrase, or missing information for a multisignature setup, may not be enough.
Two different responsibilities
With a custodial service, the provider generally controls the signing keys and maintains an account for you. With self-custody, you take responsibility for backups, device security, and the ability to recover access. Neither arrangement removes all risk. Security guidance.
Hardware wallets can isolate signing keys from a general-purpose computer, but they do not make scams or unsafe backup practices harmless. Verification on the device and protection of recovery material still matter.
Understand before experimenting
Never enter real recovery words into an educational demo. This site has no reason to request them. Learn how a wallet represents funds next in the UTXO guide.