

Most/all the main web browsers in use support kiosk mode, you’ll just need to search around for how to do it. This should get you going for Firefox
https://support.mozilla.org/en-US/kb/firefox-enterprise-kiosk-mode
Am not too sure how to get out of kiosk mode afterwards, maybe test it when you don’t have anything else running just in case.
EDIT: If you need to run kiosk mode for a specific amount of time as you say, you’ll probably need to also run a scheduled cronjob or similar to maybe kill the web browser at a specific time. I’ve seen kiosk mode for a few web browsers but they don’t usually have an option to set them for a specific amount of time.

How long has it been since the upgrade? You only get 10 days to do it easily so if it’s been less than 10 days just go to Settings / System / Recovery / Go back
https://learn.microsoft.com/en-us/answers/questions/2337049/downgrade-from-windows-11-to-windows-10-article
If it’s been more than 10 days then you’d need to do a fresh re-install of Windows 10 on that system, meaning losing the currently installed apps and data. Make sure to back up before doing that.
EDIT: Maybe I misunderstood your question - if the laptop never had Windows 10 to begin with then there’s nothing to rollback to. You’d have to do a brand new installation of Windows 10 on that system which may or may not work if the system never had the proper Window 10 drivers & whatnot.