How do the links (both Marketo and Non-Marketo) populate in the dropdown of Visited Web Page trigger or filter?
These links are fetched from any of your leads' activity logs. The values in the dropdown of the "Visited Web Page" will show if there is any lead who visited a webpage and this webpage or link is stored in the activity log of the email (may be in past).
It the expected webpage doesn't show up in the dropdown list, then none of the leads have visited that page at least once. You can create a test lead for yourself, visit the desired webpage to get this added in the dropdown.