You have to create an anchor if you need to load an entity into your scene. In order to get a ModelEntity, you need to grab it from the scene hierarchy using .children[X] subscript.

import RealityKit

class ViewController: UIViewController {
    
    @IBOutlet var arView: ARView!
    
    override func viewDidLoad() {
        super.viewDidLoad()            
        do {
            let path = Bundle.main.path(forResource: "Matrix", ofType: "usdz")!
         
            let url = URL(fileURLWithPath: path)
               
            // Scene
            let scene = try Entity.load(contentsOf: url)          
            print(scene)
            
            // Entity
            let entity = scene.children[0].........children[0] as! ModelEntity
            
            entity.model?.materials[0] = UnlitMaterial(color: .red)
            
            let anchor = AnchorEntity(plane: .any)
            anchor.addChild(scene)
            arView.scene.anchors.append(anchor)

         } catch {
             print(error)
         }
    }
}

You can also get a model this way:

let modelEntity = try Entity.loadModel(contentsOf: url)
                    
modelEntity.model?.materials[0] = UnlitMaterial(color: .red)

P.S.

I should say that you have an obvious naming error – "Matrix" vs "matrix". Also Matrix.rcproject and Matrix.usdz are not the same. To load Matrix.rcproject (Reality Composer project) use the following approach:

// .rcproject
let scene = try! Matrix.loadCircle()

let circleEntity = scene.children[0]...........children[0] as! ModelEntity

to load a USDZ model use this one:

// .usdz
let model = try! Entity.loadModel(named: "Matrix", in: nil)

But as far as I know, you do not need an RC project, so export USDZ from Reality Composer.

To load .reality file use the following approach:

// .reality
let carModel = try! Entity.loadAnchor(named: "car")
print(carModel)

arView.scene.addAnchor(carModel)

Here's your USDZ model on iOS simulator: