I am deep in this so hopefully my story will help.
There is no backup built in. This has always been super weird to me as all other production data services have it. Plus it could be a nice money maker for them. This was brought up at UC a few times and they always said no there is no plans. They are implementing (or have already) a Recycle Bin type feature where you could restore if someone deleted an entire item. Anyone got a reference here?
Its hard on us since we can have millions worth of data in a single service. Even if you assume AGOL will never lose our data, the more common issue is users accidently deleting data.
The best we have now is a Python script that runs each night as a windows batch job. It basically simulates you going to the Item page and picking Export - To GeoDatabase. With our security getting logged in via script is rather hard. So this year I switched to a script that uses the credentials from Pro. You login via Pro then Python uses that for up to 2 weeks. I have a dedicated machine that just runs this script everyday. I have to go check on it regularly. Also strangely only the owner of the service can run the script so if I am out of town or anything we just have to hope. It works pretty well until the files get to around 10 GB then it starts to fail or never complete. Most of this size is due to photos. At that point in the season I have to modify the script so that it downloads all the layers except the photos. That means I could still lose the photos which is still a big loss for us.
As for restore I would love to hear any ideas as I do not know of any way. We would have to totally republish the service from the GDB backup and all of my 123 forms. This would change the Item id which would be the biggest pain I think. The rebuild all our maps, dashboards, ..ugg. I keep all my configs in notepad so I can copy paste in the rebuild. There is an idea to upvote here to retain item id https://community.esri.com/t5/arcgis-online-ideas/keep-webmap-service-item-id-static/idi-p/932310
For now we only can use the backups to go back and look for deleted or overwrote data manually.
As the admin this is one of those that keep me up at night. I have 10 million of data on the line and I am flying with no built in backup solution. Again I have just never seen a enterprise class system with no backup before. I have spoken with Esri staff, reps, and even the product managers (even Jack I think) themselves but I cannot get anywhere. AGOL is hitting 10 years old now and is a critical system for a lot of companies so I think it is time.
I post my story in hopes others chime in here to show how important this is to you.
My script is here https://community.esri.com/t5/arcgis-survey123-questions/backup-survey-123-feature-layer/td-p/104150... . It is similar to https://www.esri.com/arcgis-blog/products/arcgis-online/data-management/back-up-data-in-arcgis-onlin... but built for daily use and a config file to download multiple services.
Thanks!!