https://arduinojson.org/v5/assistant/

{
  "id": "MillenialDIY2020LE",
  "unixTimeStamp": 1637135396,
  "BME280": {
    "temperature": 50.85,
    "humidity": 30.85,
    "pressure": 1050.85
  },
  "MPU9250": {
    "Accelerometer": {
      "accelX": 0.85,
      "accelY": 2.85,
      "accelZ": 2.85
    },
    "Gyroscope": {
      "gyroX": 0.85,
      "gyroY": 2.85,
      "gyroZ": 2.85
    }
  ,
    "Magnetometer": {
      "magX": 0.85,
      "magY": 2.85,
      "magZ": 2.85
    }
  },
  "generatedNumbers": {}
}



// Firmware

const size_t capacity = JSON_OBJECT_SIZE(0) + 5*JSON_OBJECT_SIZE(3) + JSON_OBJECT_SIZE(5);
DynamicJsonBuffer jsonBuffer(capacity);

JsonObject& root = jsonBuffer.createObject();
root["id"] = "MillenialDIY2020LE";
root["unixTimeStamp"] = 1637135396;

JsonObject& BME280 = root.createNestedObject("BME280");
BME280["temperature"] = 50.85;
BME280["humidity"] = 30.85;
BME280["pressure"] = 1050.85;

JsonObject& MPU9250 = root.createNestedObject("MPU9250");

JsonObject& MPU9250_Accelerometer = MPU9250.createNestedObject("Accelerometer");
MPU9250_Accelerometer["accelX"] = 0.85;
MPU9250_Accelerometer["accelY"] = 2.85;
MPU9250_Accelerometer["accelZ"] = 2.85;

JsonObject& MPU9250_Gyroscope = MPU9250.createNestedObject("Gyroscope");
MPU9250_Gyroscope["gyroX"] = 0.85;
MPU9250_Gyroscope["gyroY"] = 2.85;
MPU9250_Gyroscope["gyroZ"] = 2.85;

JsonObject& MPU9250_Magnetometer = MPU9250.createNestedObject("Magnetometer");
MPU9250_Magnetometer["magX"] = 0.85;
MPU9250_Magnetometer["magY"] = 2.85;
MPU9250_Magnetometer["magZ"] = 2.85;
JsonObject& generatedNumbers = root.createNestedObject("generatedNumbers");

root.printTo(Serial);