"Recently added" has the titles that have been newly added to the mangadex database by users, it could be anyone, Users without the contributor/Power Uploader/Staff role need to go through a approval queue though to reduce the number of duplicates or titles that don't belong here... I'm guessing you meant place holder titles and not chapters.
Yes, It's a mix of old and new cause multiple people are doing it on their own accord, If you find something missing you can add it to, by going through the hamburger menu on the top left and clicking the "+" sign beside the title option.
Chapters are user contributed as well, not only one guy does it, multiple people do it, the groups that scanned the manga itself, people who found the ex-license's chapters, etc., you can upload as well it fits the rule but again you'll need to go through an approval queue, the first chapter per language needs approval.
The chapter count on the site tho, I'm not sure about it...
Or do you mean the manga title count? I think that's prolly nowhere there yet
