{
  name: "test",
  isActive: true,
  items: [
   {
     id: "123",
     itemName: "testItem",
     qty: 1
   },
   {
     id: "555",
     itemName: "anotherItem",
     qty: 5
   }]
}

this.firestore.collection(<collectionName>).doc(<docID>).update({
 answers: firestore.FieldValue.arrayUnion(<AnswersObject>)
});