The style exists because a garment lying flat hides its fit. On a mannequin a shirt has shoulders, a chest and a hem that falls the way it will on a customer. Removing the mannequin keeps that shape and takes away the distraction, so the shopper sees only the product and the inside of its collar.
Done by hand it is a composite: the garment on a mannequin, then a second frame of the inside back neck, joined in retouching. Every product needs both frames in the same light and crop, or the category grid looks uneven. For how it compares with laying the garment flat, read ghost mannequin vs flat lay.
