A modern wallet isn't a pile of keys. It's one seed phrase. Twelve words become a 512-bit seed (BIP-39), and that seed deterministically grows an entire tree of keys and addresses (BIP-32). Back up the phrase once and you've backed up every address you'll ever use.
Path m / 44' / coin' / account' / change / index. The ' marks a hardened step. Hover an address to see its private key, all from the phrase above, nothing stored.
The same phrase always rebuilds the exact same tree, in any conforming wallet. Lose your device, type the words into a new one and every address comes back.
Add a passphrase and you get a completely different tree from the same words, a plausible-deniability vault. Forget it and that money is gone forever.
A parent xpub can normally derive every child public key, and worse, that xpub plus one leaked child private key exposes the parent and all siblings. Hardened steps cut both links. That's why account-level paths use the apostrophe.