I am curious about how exactly aliases work. like what does the checkmark do exaclty, and who dictates what being changes, like with the alise of 2girls to 2_girls, even though 2girls has roughly 30k images with it and 2_girls only has 400.

That is a question for the mods my man, but near as I can tell, the system reads the two similar aliases as different, and therefore chooses the images accordingly

On e621.net is discussed by the users in the forum and then decided whether an alias is changed or set. There was some time ago, for months, a discussion about Cuntboy and Dickgirl, who should get new names. Here I guess the admin decides.

If I'm not mistaken, alias work like this. Titis, boob, breasts, ...... no matter what word you use in the search, the search probably shows you breasts, since they are fused together in the word since they have the same meaning. They should simplify things and in e621 "Cuntboy" they should replace abusive words. Can not guarantee that it is so.