It's entirely up to a manufacturer to decide how to designate the various products it produces. Airbus has chosen to designate its models with the letter "A," followed by three digits. So, for example, when one says "Airbus A220," one is identifying the manufacturer (Airbus), and the model number (A220). You may think it superfluous to include the letter "A" in each model designation, and you may be right about that. But it's Airbus' decision to make, not yours or mine

Embraer uses "E" at the beginning of some of its models, and "ERJ" for others.