“AR” refers to a pattern of rifle. Any manufacturer can produce complete rifles or various components and accessories that fit the AR pattern. Those components are interchangeable. This allows you to customize your rifle to your current and future needs simply by swapping parts, rather than needing a gunsmith to machine and fit them.

“AR” is to guns like “Pickup truck” is to cars. A pickup might be a simple, ordinary vehicle. It might be a heavily customized rock crawler. It might be a lowered and styled cruiser. It might have a ladder rack, or have the bed swapped for a maintenance truck, or flatbed. It might be converted to repossess vehicles, or fight brush fires. Likewise, an “AR pattern rifle” refers only to the most basic structure of the rifle rather than a specific gun.