Second link of firefox's results after looking up "mangadex" looks like a pretty obvious fake because iv used the site before, just posting this to let yall know if you couldn't tell if you stumbled upon it yourself but it dosnt seem too harmful as there are no login pages or anything (marked as nsfw because of the ad in the corner but it really isn't nsfw)
Because www is just a subdomain. It can be anything. it could be "mail.site.com", "xxx.site.com", "home.site.com", "wwwwwwwww.site.com" etc. The www subdomain is just what most people used to point to where their world wide web site html files were held back in the day. The rest of the domain was used for other things.