There are two types of exclusive content.
First one is the regular updates of the site that are connected by an ongoing story arc. You can access those by clicking on the Episode Guide.
Second one is irregular bonus stuff. Some are experiments, some are works by other artists that have contributed to the world of the Project in one way or the other. Click on the Extras Guide to access those.
To access exclusive content you need to be a member of the Patreon campaign. The page will ask you to login with your Patreon credentials.