Does anyone know how to upload data to ArcGIS Hub or what is the difference between Hub and Open Data?
Add data to your site—ArcGIS Hub | ArcGIS
Short answer is you need to have your items in groups so you can use those groups on your site in Hub. If you choose to, you can enable Open Data on your ArcGIS Online organization and use a group designated as Open Data. These Open Data groups can be included/federated on Hub Sites that are outside of your organization so make sure you are comfortable with sharing your data in this manner before doing it.
Hub was formerly called ArcGIS Open Data but as of 2016 the application is called Hub.