Do users and passwords created by
useradd command in linux also register them with same creditionals in MySQL database?
useradd creates a local user that can potentially log in to a Unix/Linux system and execute commands from the command line, or may have access to FTP services, etc.
CREATE USER, which is a mySQL command, creates user accounts for the MySQL database only. One is not related to the other in any way. Permission changes applied to a Linux user do not reflect on MySQL users, and privileges granted to MySQL users apply only to MySQL tables and schemas.
The comparison is much like comparing apples with bricks.