Login Screen Before OS Loads?
Boot Process
Before you can be presented with a login screen, the OS must be booted.  Granted, there can be &quot;login items&quot; which are apps or helpers that can be loaded after a user logs in, but the bulk of the OS (this includes the kernel and related drivers) has loaded by the time you get the login screen where you enter your username and password.
To simplify it, if you can log into your Mac remotely before you've entered your login credentials, your Mac has booted completely.  As far as User applications that run in the GUI or get started by launchd (LaunchAgents) those will be started after you've successfully authenticated.  These aren't really part of the OS, but part of the userland environment - they're not required for macOS to boot and run, but they are required for the user experience.  You will see these loading on the screen in verbose mode.
The Eclectic Light Company has an excellent flowchart (below) as to the complex nature of the Apple boot process - but one thing to notice is there are a couple places for password input long before the system actually starts booting the kernel.

Firmware Lock
Mac computers both laptop and desktops now have heightened security and provide for a firmware lock.  This protects your device in that it prevents starting up from any internal or external device that wasn't designated a startup disk.
FileVault
FileVault is Full Disk Encryption of your storage device.  It's unlocked with your login password so it can appear that the boot process hasn't finished until you login (I suspect this is what you're seeing).

Typical Login Screen:

Firmware Lock Screen: