You need to do a second and third .map inside our first map in order to return both rooms:

{props.house.map(house => {

     return (
         <React.Fragment>
             <Card>
                 <Card.Body>
                     {
                         house.rooms.map((room) => (
                             room.map((device) => (
                                <Card.Title>{device.device_name}</Card.Title>
                             )
                         )
                      }
                 </Card.Body>
              </Card>
          </React.Fragment>
       )
     }
  })}

You have 2 levels in your array, when you do props.house.map, you only get the first level object: dwelling. If you want to list all the rooms of a dwelling, then you need to make a second map like this:

props.house.map(dwelling => {
  dwelling.map(room => {
    console.log(room.devices[0].device_name);
  }
})

When I use map() to return the names of devices in my database, it only returns the first name.
I am not very confident with JavaScript so this may be a simple issue.

Here’s my database:

[{
   "dwelling_id": 1,
   "rooms": [{
       "room_id": 1,
       "room_name": "Living Room",
       "devices": [{
          "id": 1,
          "device_name": "Google Home",
        }]
      },
      {
        "room_id": 2,
        "room_name": "BedRoom",
        "devices": [{
           "id": 2,
           "device_name": "Smart Bulb 1",
        }]
       }...

Here’s the code to return both Google Home and Smart Bulb in separate Bootstrap Cards:

{props.house.map(house => {
     return (
        <React.Fragment>
           <Card>
              <Card.Body>
                 <Card.Title>
                   {house.rooms[0].devices[0].device_name}
                 </Card.Title>
              </Card.Body>
           </Card>
        </React.Fragment>
     )}
})}

If I do this:

// If I change this...
{house.rooms[0].devices[0].device_name} 
// to...
{house.rooms[1].devices[0].device_name}

My code only returns just Smart Bulb.
Note that I want to return both Google Home and Smart Bulb in two separate Cards.
Any help would be greatly appreciated, thank you!