$15 AOC C27G2U monitor stand is a proprietary part (model number Q37G10778030010SCX) designed for the C27G2 monitor. This stand offers a wide range of ergonomic adjustments, including height, tilt, swivel, and pivot, which is a key selling point of the C27G2 monitor. The "x2" in your request indicates you will need to purchase two of these stands, doubling the cost.
$15 MSI MAG401QR is designed for a large, 40-inch ultrawide gaming monitor. This stand is specifically for the MAG401QR monitor and offers tilt and height adjustment. It does not support swivel or pivot, which is an important consideration. It is a very sturdy stand required to support the large and heavy ultrawide screen.
$20 ASUS ROG Strix XG309CM is a part for a popular ultrawide gaming monitor. The stand for the XG309CM is known for its sturdy build and extensive adjustability, which includes height, tilt, and swivel. The design is a notable feature, with the signature ROG aesthetics and a tripod socket built into the base for mounting a camera or projector, which is a feature unique to this model.