db={"ABC":[{"_id":1,"ABC_Action":234,"Name":"Naveen","Age":30}],"DEF":[{"_id":1,"Action":456,"Name":"Manish","Age":33}]}

db.ABC.aggregate([{$lookup:{from:"DEF",let:{"abc_id":"$_id",},pipeline:[{$match:{$expr:{$eq:["$$abc_id","$_id"]}}}],as:"def_objects"}},{"$addFields":{"def_objects":{"$first":"$def_objects"}}},{"$project":{"_id":0,"ABC_Action":"$ABC_Action","Name":"$def_objects.Name","Age":"$def_objects.Age"}}])