for the expansion dlc you'll have to download the file first just like any other itch.io game, then unzip/extract the files like any other game, but then go into the unzipped file itself, not the original game file so that you can see the dlc.rpa file. then while having this open, open up the original unzipped base game files until you see the game folder inside of it. navigate back to where you found dlc.rpa and drag that specific file (not the full folder) into the game folder inside of the unzipped base game files.
i hope this helps, i can take screenshots and stuff and circle things if you're still confused :)