* The way that e.g. macOS FileVault works, where you enter your password in EFI, before the machine boots and before the OS is loaded.
* The way that e.g. iOS works, where there is an unencrypted partition containing the OS (mounted read-only), and an encrypted partition containing the data. The OS boots off the unencrypted partition, then you have to enter your password to unlock the data partition.
* The mode you're describing, where the unlocking happens automatically if and only if nothing has been tampered with (the encrypted hard drive hasn't been swapped into another machine, etc). With this mode you're relying on the fact that when the OS boots it won't let you do anything without entering a password, even though the disk is already unlocked. Assuming there are no bugs in the login screen, no way to physically read the contents of RAM (where the encryption keys are), and no way to access the contents of the TPM, this approach is just as secure.