Managed runs through AdsNet hosting and workflow. Self-hosted is set up on your own website and Google Ads account.
What is means in simple terms is that if you prefer Managed account, this means AdsNet will host your landing page or website and can also have your Google Ads setup on our MCC and you pay Google through us for advertising budget, while Self hosted, means that a website is hosted on your own server and the Google ads are on your own account.
NB: Landing pages can not be hosted on your own server or domain.