JSFiddle - React, Tailwind, and code Playground

JavaScript

const data = {
   "questions":[
      {
         "id":1,
         "questionHeader":"Some question header",
         "questionText":"Some question text?",
      },
      {
         "id":2,
         "questionHeader":"Some question header",
         "questionText":"Some question text?",
      },
   ],
   "answers":[
      {
         "id":1,
         "questionId":1,
         "answer":"Some answer",
      },
      {
         "id":2,
         "questionId":1,
         "answer":"Some answer",
      },
      {
         "id":3,
         "questionId":2,
         "answer":"Some answer",
      },
   ]
};

const newItem = data.questions.map((t1) => ({
    ...t1,
    ...data.answers.find((t2) => t2.questionId === t1.id),
  }));
  console.log(newItem);