for some reason the Game Maker Studio 2 source code for Hatred Island only has the yyz file? if I'm right it should contain all the assets.
for example: take the github source code for Undertale: https://github.com/marciniuk/undertale. it has folders for the assets.
could you change it to a zip file, containing all the assets?
for some reason the person that sent me this question hasn't tried to open the project. yyz files are compressed projects that contain all the data and assets.