Full disk encryption (FDE). You provide the password at boot and either you can or can't decrypt (typically the key itself is derived from the password). You can also do this without FDE by doing the same thing but keeping the password around in memory if you're trying to avoid prompting them.

Modern machines work slightly differently. The key material is stored in a TPM which is a separate processor & dedicated memory that is purpose built to withstand physical and electrical attacks. Apple devices specifically have a complicated key wrapping scheme (protected by your pincode or password) to make certain files accessible/inaccessible depending on the policy defined (available after first unlock, available only when unlocked, available always, & a fourth one I forget). Your password is just used for protecting the underlying keys but the device actually generates strong key material that's used to protect all on-disk contents regardless of a password being present IIRC.

If you're talking about the password database for local login & whatnot, that was available without even having FDE by using PBKDF2 or similar to securely hash the password. That way you only store the hash & leaking that file doesn't mean that someone can reverse that back to get your password.