The Raspberry Pi has traditionally used the pi username. In response to some countries placing restrictions on default credentials on devices, Raspberry Pi have now dropped that default user. Instead during first run you will be prompted to enter a username as well as to change the password.
In the video below I discuss the pros and cons and discuss whether it really does provide additional level of security and whether that is worth paying for the lack of convenience.
Transcript: Raspberry Pi default Username - Video Transcript
This video on the Penguin Tutor YouTube channel shows how I modified software to allow it be used on the new version of the Raspberry Pi OS without the default pi username.
For more details about how username and password security work on Linux systems see the following guides: