An organization administrator or the current owner of a site application item can transfer their site and its content to a new owner.
Ensure that the site is shared with a shared update group and the new owner is a member of that group. Site owners can edit and manage the site in the workspace. The new owner must have the privileges to manage content.
Learn more about changing the owner of an item in ArcGIS Online
Change group ownership
An organization administrator or the current owner of a group can change the owner of a group that is shared with the site. When reassigning group ownership, you do not need to remove items from groups that were added to the site using the content catalog.
Learn more about editing group properties and settings in ArcGIS Online