The Nintendo Switch attachment that holds two Joy-Con controllers together is the Joy-Con grip.
The Joy-Con grip is included with the original Nintendo Switch and the Nintendo Switch OLED Model. Each Joy-Con slides into one side of the grip, placing the left and right controllers beside one another in a shape resembling a conventional gamepad.
The grip does not charge the controllers. A separate charging grip, sold independently, adds charging contacts and must be connected to power. This distinction is a common source of confusion because the two accessories look similar.
The Joy-Con grip differs from the Nintendo Switch Pro Controller, which is a separate wireless gamepad with its own fixed controls, battery, and body. The grip simply provides a more comfortable way to use the detachable Joy-Con pair.