db={"EngineeringStudent":[{"studentId":1,"name":"Tom","roll":12,"dob":2001,"yom":2022},{"studentId":2,"name":"Tom2","roll":12,"dob":2001,"yom":2022}],"ScienceStudent":[{"studentId":3,"name":"Tom3","roll":12,"dob":2001,"yom":2022},{"studentId":4,"name":"Tom4","roll":12,"dob":2001,"yom":2022}]}

db.EngineeringStudent.aggregate([{$facet:{EngineeringStudent:[{$match:{}}],ScienceStudent:[{$limit:1},{$lookup:{from:"ScienceStudent",pipeline:[],as:"ScienceStudent"}},{$unwind:"$ScienceStudent"},{$replaceRoot:{newRoot:"$ScienceStudent"}}]}},{$project:{users:{"$concatArrays":["$EngineeringStudent","$ScienceStudent"]}}},{$unwind:"$users"},{$replaceRoot:{newRoot:"$users"}}])