Looks like you have a 2018 Mac mini? Click on the Apple logo top-left on the menu bar and click About this Mac.
The Intel 2018 Mac mini has the T2 Security Chip which is basically an iPad processor. The disk is connected to the T2 which provides the disk controller and the Secure Enclave chip that is a write only black box to hold all the secrets such as private keys. Once you write a private key to the Secure Enclave there is no way to read it back. All you can do is send a public key and the chip responds Yay or Nay on finding a match. If a match is found then things get unlocked. The Intel w/T2 Macs and the new Apple Silicon M1 / M2 Macs are ALL hardware encrypted ALL the time. Turning on FileVault on these systems is very quick because it doesn't actually encrypt your entire SSD as it's already encrypted. But it does generate a recovery key which you can save in iCloud. That will allow you to reset your password should you forget it. Otherwise you won't be able to access the files on the disk. When you boot up the login screen appears very quickly, after you enter your password you will see a progress bar. That's the actual operating system booting up. What you saw is known as the pre-boot authentication screen and it looks just like the real thing but the system is not booted yet. Once it boots it uses SSO (Single Sign-On) to log you into the Desktop.
So to summarize, your Mac is fully encrypted ALWAYS and you can't turn it off.
There is a way to encrypt files and folders by creating a disk image and saving the password for it inside your keychain (or just never forgetting it if you don't save it in the keychain).
Create a disk image using Disk Utility on Mac - Apple Support
This is ideal if you are going to copy confidential files stored inside an DMG off the internal Mac SSD to external storage of any kind.
