On what exactly am I supposed to base my decision on?
I don't know which metada is affected, what value LrC has or what is the other value. It would be great to have some of these informations, or no warning at all.
This notification is a useful reminder in situations where you KNOW why and how, and where you EXPECT that, the metadata will have been altered by some means other than Lightroom Classic.
In situations where you ensure nothing is happening to these files on disk, except under the control of Lightroom Classic - where LrC is fully earning its keep as manager for an image library - the warning will never arise.
So something has arisen to alter the external metadata, so as to differ from the internal metadata of your Catalog. Only you have any knowledge of what that may have been. I would agree that maybe a summary of what metadata are changed vs what are the same could be helpful in principle, when considering a single image, but imagining the not uncommon situation where many images show this warning at the same time, this would be very unwieldy. Only your own understanding of what has happened can guide the proper action to take.
Perhaps you know that anything done externally can be safely discarded; perhaps you have used more than one Catalog where these image files are imported, and both have written their own changes (in mutual conflict).
Perhaps you know that another utility has revised certain metadata fields that LrC would not itself have ever altered, and then your best course may be to first write out all current LrC metadata from the Catalog (in order not to lose those latest edits, but without overwriting the external fields you've separately updated). These are the workflow steps sometimes used e.g. when externally altering (with exiftool) specific Lens information, different from how that was initially interpreted by LrC. Then everything can be read back in together.
But if this external change of metadata is a complete surprise to you, you can either ignore the warning or else I'd probably just presume that the Lr Catalog (that you are currently using, if more than one exist) will tend to be the more definitive. Perhaps these images were separately opened into ACR or Bridge, and different metadata got written from there. Perhaps you have a previous version Catalog and also a later version Catalog that have both referenced the same files in common. Perhaps you are also involving another DAM.
All those scenarios are problematic and messy IMO, compared to the straightforward black-box approach of one LrC Catalog being the sole assigned front-end for accessing, and the sole means of managing, a whole library of images - including all of the storage folders and source files concerned. And the user leaving that stuff otherwise completely alone!
It looks like you've made some changes (metadata and/or edits) to the file inside LrC and in another application. Unfortunately, LrC has no way of knowing what the specific external changes are. Therefore, you need to decide whether to keep the LrC changes or import the externally applied changes. If you can't recall what the external changes are or even having made them, then using the 'Overwrite Settings' option is probably the safest.
In future, if files are already in the LrC catalog, then try to avoid making changes to them in other applications.
First, I did not change anything to the images externally: the warning appears on a series of archived images I haven't touched in years.
Second, what is the criteria used by LrC to pop that warning? If it doen't compare metadata and/or edits how can it tell there have been changes, and if it does, why can't it tell which ones?
You may not have changed anything yourself, but something (perhaps on the OS level) did; the metadata found was newer than what Lightroom Classic provided so you got that warning.
As for what to do, generally speaking, I'd update (Import settings) into LR so it's database (catalog) is in sync with the images.
LR has always produced occasional spurious metadata-conflict warnings since time immemorial, but there are more reports about it in LR 10, and LR 10 introduced (more) bugs in determining when the catalog metadata hadn't been written back to disk:
Adobe has never given any signs of prioritizing these bugs.
LR has never disclosed precisely how it determines the disk metadata is up-to-date. But experimentation shows that it uses the file's last-modified time to identify files that have changed externally, and then it compares the metadata field-by-field to see if in fact the fields have changed. Retesting this right now, I've seen it work sometimes and sometimes not.
If you're sure that you haven't used an external app on a file, the best you can do is click Overwrite Settings, which will write the catalog metadata to the file and (maybe) cause LR's metadata status indicator to change to Up To Date.
In my own catalogs, I set the option Catalog Settings > Metadata > Automatically Write Changes To XMP. If I see metadata-conflict warnings, I immediately do Metadata > Save Metadata To File / Overwrite Settings, except in the rare case when I've used an external app to modify the photo's metadata.