Hi,
That is very extensive depending on what you are looking to accomplish. Without knowing the specifics, you can use the WebGISDr tool, to back up your Enterprise (see link below). You can go out to the admin endpoints in your system and do exports of Server, Portal, ect. and manually put them in an S3. Finally, there is an option to use Python API and do exports and use command line to move content from a server to the cloud. Pretty sure there might be a few other ways to go about it but those are first thoughts that come to mind.
https://enterprise.arcgis.com/en/portal/latest/administer/windows/create-web-gis-backup.htm
