The letter @, if we can call it a letter, was used in the Middle Ages by copyist monks as an abbreviation of the sound "ad". So they merged the "a" and the "d" to give a letter like this:
Then, over time, the "d" bar was removed to give @.
Today, we say "at sign" because, when the printer was invented, the printed characters were placed so that the lowercase letters were at the top of the table and the capital letters and special characters at the bottom. The at-sign was therefore at the bottom of the table and, to find their way around, the printers called this letter: a-rond-bas. Because it was at the bottom of the table and there was a circle around the "a". Over time, this appellation has changed to become Arobase.
Thanks for reading.