Understanding SharePoint is always difficult after many years of SharePoint I'm still not always understanding all of it. ;)
The quick instruction on creating a web part that shows older than X documents:
First go to the SharePoint admin center.
Go to search
Click on manage Search Schema
Find Refinabledate00 managed property ( check in the Mapped Crawled property that it hasn't been mapped yet, otherwise go for 01, or 02 or .....)
Edit Refinabledate00 and map the property in the Mappings to crawled properties section to the ows_Modified crawled property.
Now to trigger a search crawl you will need to go to the site . In the modern sites there isn't much you can do other than go to a library -> Library settings -> Advanced settings -> Reindex Document Library.
Go to a modern page where you want to add the web part ( e.g. /SitePages/Home.aspx)
Click on Edit (top right)
Click on a + to add web parts
Select the HighLighted content
When the webpart appears click on the pencil to edit the web part properties.
In the filter and sort section:
Select Managed Property and select RefinableDate00. Select Before and supply a date.
Now you should find that the results will appear at some point. But as this depends on SharePoint search there may be a delay.