Many agencies, such as the General Services Administration, are involved with transitioning from one president to the next.
These are only examples of resources available.
Often abbreviated to EOT.
At the end of a U.S. Presidential term, a team of volunteers nominate top level U.S.Eexecutive Branch domains to archive.
According to the page on EOT web crawl background:
"White House websites are Presidential records. The official files that make up a Presidential administration's website are preserved in our Executive Office of the President Electronic Records Archive. In order for the public to easily access the websites, the National Archives has taken an additional step to "freeze" the White House websites and make them available online. Because the archived websites are hosted by the National Archives and are historical material, they are no longer updated. Any broken links (internal or external) will not be updated.
"The archived websites are an excellent resource for photographs, speeches, press releases, digital data and other public domain records.
"The End of Term Web Archive is a collaborative initiative that collects, preserves, and makes accessible United States Government websites at the end of presidential administrations."