The Join Features tool seems to have worked:
Add a feature layer to a web map by using the Join Features tool.
Note: I couldn't find the Content --> Analysis button in the new Web Map Viewer (Beta).
I had to use the classic Web Map Viewer instead.
Create an application to authorize HTTP requests to AGOL
Query an ArcGIS Online FC via an HTTP ...
I'd say join field is the tool you need. To test it I created 3 identical tables named "A","B","C":
after running this script:
for i,letter in enumerate("ABC"):
first = letter
arcpy.AddMessage("Joining %s to %s" %(letter,first))
You did create a join, not a relation.
Joins and a relations are different things. A join (layer based) does add attributes from another layer based on a 1:1 or 1:0 cardinality relationship, but in the described case you have a 1:n relationship (1 project : n trees).
The behavior you need can be achieved via Project > Properties... > Relations. Add a ...