Arcgis Portal
https://portal.laquintaca.gov
https://gis.la-quinta.org/portal/apps/sites/#/la-quinta-open-data (Actual site URL, hosted by your local machine, not ESRI)
Your server, your machine, your hard drive, ESRI will not charge you no matter how much data you uploaded.
https://gis.la-quinta.org/portal/apps/sites/#/la-quinta-open-data/search
Arcgis Hub (online)
https://opendata.laquintaca.gov/
https://la-quinta-open-data-laquinta.hub.arcgis.com/. (actual site URL, hosted by ESRI)
You are using ESRI's server, machine, using ESRI's hard drive, RAM memory, CPU time, for example, upload 1 parcel layer(about 20k home address) will cost you $300 per months. If you just reference the layer on your arcgis server will be a lot cheaper as storage rate a few dollar per month for 10GB. Check out ESRI rate at ESRI pricing table.
https://doc.arcgis.com/en/arcgis-online/administer/credits.htm
Understand how ESRI will charge you
https://transparentgov.net/cleargov1/972/how-much-esri-will-charge-you-on-arcgis-online?show=972#q972
https://opendata.laquintaca.gov/search
Understand the relationship between ArcGIS Enterprise and ArcGIS Online
ArcGIS enterprise VS ArcGIS online
https://proceedings.esri.com/library/userconf/proc18/tech-workshops/tw_1762-276.pdf
Functionality Differences between Online and Portal
https://community.esri.com/t5/arcgis-online-questions/functionality-differences-between-online-and/td-p/168135