Good afternoon,
In ArcGIS Pro I can create a join between a hosted feature class that belongs to the Organization and a table that is stored in my content space. But when I try to publish it to AGOL, it returns the following error message "00226: In-memory joins are not supported"
Known limit: https://pro.arcgis.com/en/pro-app/latest/help/sharing/analyzer-error-messages/00226-in-memory-joins-...
I would just perform the join in AGOL.