Probably in the fine print of the license agreement which I think is included in the box of these devices–which I hope the previous owner(s) passed on to you)–or a link to an Apple web site. Or perhaps best to contact Apple Support for this? Ensure you tell them you are asking about a device which had a previous owner, and if or if not in Apple Care.
From 2018 - but is here is an explanation from the Apple forums
I would also add that as far as I can tell, the warranty is valid internationally - but without the paid-for Apple care, it is limited. This from from here
If you have a used Apple device still covered under the Limited Warranty or AppleCare, you can have it repaired globally at an Apple Store or an Authorized Apple Repair Service Provider. Turnaround time will vary greatly.
If an iPhone cannot be repaired but needs to be swapped out, that could take up to a week for a replacement to arrive. Depending on where the iPhone was originally purchased, you may have to wait until you return home.
Since you purchase used devices, you might want to contact AppleCare to ensure the coverage is still valid! The previous owner can cancel it and receive a pro-rated refund. I have seen this at my former store in the past.