The built-in memory of a computer typically refers to RAM (Random Access Memory). RAM is a type of volatile memory that the computer uses to temporarily store data that is actively being used or processed. It provides fast access to the data the computer needs to perform tasks, and its content is erased when the computer is powered off.

The "@" symbol is used to separate the username of an email address from the domain name of the Internet Service Provider (ISP) or email service provider. For example, in the email address "username@example.com," "username" is the username of the email account, and "example.com" is the domain name of the email service provider, separated by the "@" symbol.