[{"Name":"Test","Location":"Whatever","customerServices":[{"id":"test","cusId":"test","adr":"Adr 1","serviceCounty":"Center","area":"village"},{"id":"test","cusId":"test","adr":"adr2","serviceCounty":"West","area":"city"},{"id":"test","cusId":"test","adr":"test","serviceCounty":"West","area":"test"}]}]

db.collection.aggregate([{$addFields:{serviceAreas:{"$reduce":{"input":"$customerServices","initialValue":"","in":{"$concat":["$$value","$$this.serviceCounty",", ","$$this.adr",", ","$$this.area",", "]}}}}}])