If multiple people are working on a manga or anime, how can the art style be copied so well without any style changes? Like can everyone replicate art styles exactly with no mistakes? In most American cartoons I watch, if multiple people work on an animation, you could definately tell if someone different drew it. Maybe I’m not observing enough or this question is stupid.
In the case of animation, you can actually tell the difference, though usually it's pretty subtle. There are otaku who make the close study of animation to identify particular animators (as well as other very discerning details) their particular hobby. This is known as sakuga.
But also, the way the work is divided encourages consistency, like in the case of manga, they might have an assistant do all the backgrounds while the main artist draws the actual characters. So with manga, it's usually always the same artist drawing the same category of thing, so of course the style will always look the same. In the case of animation, lead animators draw the key frames, and in-betweeners usually fill in the less important frames in between.
Given enough time, really good artists can imitate another artist's style pretty accurately. Only a very discerning eye might notice differences. Many of the drawing inconsistencies you might see in animation are due to not giving the artists enough time to produce really quality work.