Query that flattens many tables, but now want to concatinate one set of strings into a single string

I have an SP that flattens a bunch of tables. I recently refactored so that instead of a single description in one table I now have a one to many relationship with a descriptions table. I can inner join and get the descriptions on many rows, but I'd rather just concatenate the descriptions into a single field. I'm trying STRING_AGG, but I don't think it works for my purposes. Here's the working query that just gives me each description on its own row and commented out what I tried to concatenate (adding a comma to the end of the line above as well). I don't really grok SQL well enough to even know how to proceed . . . Select P.ID as ProjectID, P.Code, P.MOAProjectname, P.Contractorname, P.Ownername, M.ID as PeopleID, M.Firstname + ' ' + M.Lastname As Fullname, R.ID as ReportID, R.Reportnumber, R.Reportdate, R.Weather, R.Temperature, R.Presentatsite, R.Estimatedcomplete, R.Progress, R.Trades, R.Note, D.DivID, D.DivName, O.ID as ObsID, O.Description, O.PicCaption, O.Opendate, O.Closedate, O.ImagePath, O.ObservationNum --STRING_AGG (T.Description,', ') AS Dsc From Observations as O inner Join Reportinformation as R on O.ReportID = R.ID Inner Join Descriptions as T on O.ID = T.ObsID Inner Join Divisions as D on O.Division = D.DivID Inner Join MOApeople as M on M.ID = R.MOApeopleID Inner Join Projectinformation as P on P.ID = R.ProjectID Order By P.Code, O.Division, O.ObservationNum I am guessing that I could create a temp table that does the aggregation, and then join that table into my main query. Is that the right direction to pursue?