db={colA:[{"day":1,"title":"Lorem ipsum dolor sit amet","data":[{"type":"body","content":"Lorem ipsum dolor sit amet"},{"type":"content_list","content":[{"id":"6312b5bd0fb68141c6bdc4d0",},{"id":"6311c6c50fb68141c6b97710",},]},{"type":"body","content":"Lorem ipsum dolor sit amet"},{"type":"content_list","content":[{"id":"6312b5bd0fb68141c6bdc4d0",},{"id":"6311c6c50fb68141c6b97710",},]}]}],colB:[{"id":"6312b5bd0fb68141c6bdc4d0","title":"AAAA","date":"21-3-2022","duration":"10 min"},{"id":"6311c6c50fb68141c6b97710","title":"EEEE","date":"21-3-2022","duration":"10 min"},]}

db.colA.aggregate([{$unwind:"$data"},{$lookup:{from:"colB",localField:"data.content.id",foreignField:"id",as:"data.contentL"}},{$set:{"data.content":{$cond:[{$eq:["$data.type","content_list"]},"$data.contentL","$data.content"]},"data.contentL":"$$REMOVE"}},{$group:{_id:{day:"$day",title:"$title"},data:{$push:"$data"}}},{$project:{_id:0,day:"$_id.day",title:"$_id.title",data:1}}])