Database
db={users:[{"_id":"aandtv10@gmail.com","updateTimestamp":"2022-07-19T11:59:18.029Z","userConsent":null,"firstName":"Aand","lastName":"Aa","fullName":"Aand Aa","orgRootName":"Black&Veatch","relationType":"Contractor"}],documents:[{"_id":ObjectId("62e9296bf06b3bfdc93be0da"),"lastUpdated":"2022-08-02T13:40:59.265Z","email":"aandtv10@gmail.com","userName":"Aand Aa","docType":"dose2","administeredTimestamp":"2022-10-02T11:10:55.784Z","uploadTimestamp":"2022-08-02T13:40:59.265Z","manufacturer":"Novavax"},{"_id":ObjectId("62e911cf5b6e67c36c4f843e"),"lastUpdated":"2022-08-02T12:00:15.201Z","email":"aandtv10@gmail.com","userName":"Aand Aa","templateName":"Covid-Vaccine","docType":"dose1","exemptType":null,"administeredTimestamp":"2022-08-23T17:16:20.347Z","uploadTimestamp":"2022-08-02T12:00:15.202Z","manufacturer":"Novavax"}]}
Query
db.users.aggregate([{$match:{_id:"aandtv10@gmail.com"}},{$set:{data:"$$ROOT"}},{$lookup:{from:"documents",localField:"_id",foreignField:"email",as:"documents"}},{$project:{_id:0,data:1,documents:{$reduce:{input:"$documents",initialValue:[],in:{$concatArrays:["$$value",[{k:{"$concat":["$$this.docType","_administeredTimestamp"]},v:"$$this.administeredTimestamp"},{k:{"$concat":["$$this.docType","_manufacturer"]},v:"$$this.manufacturer"}]]}}}}},{$replaceRoot:{newRoot:{$mergeObjects:["$data",{$arrayToObject:"$documents"}]}}}])