[{_id:1,posts:[{"platform":"instagram"},{"platform":"tiktok"},],full_name:"John Doe",},{_id:2,posts:[{"platform":"youtube"},{"platform":"tiktok"},],full_name:"Mamed Mamedov"}]

db.collection.aggregate([{"$project":{full_name:1,"platforms":{"$reduce":{"input":"$posts","initialValue":[],in:{$concatArrays:["$$value",["$$this.platform"]]}}}}}])