Adsense Ads load directly from Google servers so you cannot control them as they are hosted elsewhere. Only the items that you host are in your control.
If a site does that, it may be because they are using some form of lazy loading. But doing it for Ads may be against Google ToS. Also, it is a very bad idea to prevent site content loading before a third party script as Google Pagespeed tool specifically suggests to delay load all scripts so that content can load first.
You could try an Ad above header if you want Ads to show first.