The h5u file in the archive is also an archive; when moved to the UserMODs folder, it becomes a mod and affects the game.Here's how it works: when the game starts, it unpacks all the archives it finds in the data folder (with the .pak extension) and in the UserMODs folder (with the .h5u extension), as well as in the Maps folder (with the .h5m extension). All the files it finds there are placed into a single structured database, and there may be situations where two files have the same name and path in this database (two different archives with "overlapping" content; the files inside are not distinguished by their names, but the files themselves are different). In this case, the game gives priority to the newer versions of the files and uses them when loading. If you "feed" the game modified versions of the files, that's what constitutes a mod.
P.S. There might be a small issue: the large hero image for the duel window needs to be corrected separately.