how do I merge the tabs of two open Preview windows if I have more than two open?
Maybe I misunderstand your question, but here's what I think you're asking:

With two (or more) open windows in Preview (macOS 10.15.x &amp; probably other recent versions):

In the menu bar, while one of the Preview windows is in focus: 


click the menu bar item labeled Window
select the Merge All Windows item from the drop-down menu
all open Preview windows are merged into one window, and each (former) window is now a tab in the single Preview window 


You can also drag tabs between windows in Finder. With two open Finder windows, and both windows set to View -> Show Tab bar, just drag a tab in one window into the tab bar area in the other window. 

AFAICT, it works the same my browser windows do. 

This may all seem a bit confusing. If so, here's a video that may explain it more clearly.