if i know mega username can is see main archive

If you have the username for a MEGA account, you cannot access the main archive or any stored files unless you also have the necessary permissions, such as the password or decryption key for that specific account or folder. MEGA takes user privacy and security seriously, and access to files is restricted to those who have the required credentials. If you are trying to access someone else’s files without their permission, it is important to respect their privacy and not attempt to do so.