Create a horizontal plane detector and replace the surface with an image

ViewController.swift

import UIKit
import ARKit
import SceneKit


class ViewController: UIViewController, ARSCNViewDelegate {

    @IBOutlet weak var sceneView: ARSCNView!
    
    let configuration = ARWorldTrackingConfiguration()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        // set delegate so renderer fn gets called
        sceneView.delegate = self
        
        // set debug options using an array
        sceneView.debugOptions = [ARSCNDebugOptions.showWorldOrigin, ARSCNDebugOptions.showFeaturePoints]
        
        // type of plane detection
        configuration.planeDetection = .vertical
        
        // run, requests camera access first run
        sceneView.session.run(configuration)
    }
    
    // create surface, in this case grass aka "turf", but could be anything depending on image
    func createTurf(planeAnchor: ARPlaneAnchor) -> SCNNode {
        // create scene node
        let turfNode = SCNNode(geometry: SCNPlane(width: CGFloat(planeAnchor.extent.x), height: CGFloat(planeAnchor.extent.z)))
        
        // now set node properties such as material and position
        turfNode.geometry?.firstMaterial?.diffuse.contents = UIImage(named: "ocean")
        turfNode.geometry?.firstMaterial?.isDoubleSided = true
        turfNode.position = SCNVector3(planeAnchor.center.x, planeAnchor.center.y, planeAnchor.center.z)
        
        // use extension to rotate 90 degrees about x axis <--x-->
        turfNode.eulerAngles = SCNVector3(-90.degreesToRadians, 0, 0)
        return turfNode
    }
    
    // This delegate fires when an anchor is added whenever an ARAnchor was added to the sceneview. An anchor encodes position, size and orientation of something ... the surface in this case.
    func renderer(_ renderer: SCNSceneRenderer, didAdd node: SCNNode, for anchor: ARAnchor) {
        print("New surface detected")
        
        // notice that we {return} early if we don't have an anchor
        guard let planeAnchor = anchor as? ARPlaneAnchor else {return}
        
        // ... otherwise we proceed creating and adding the scene node
        let turfNode = createTurf(planeAnchor: planeAnchor)
        node.addChildNode(turfNode)
    }
    
    func renderer(_ renderer: SCNSceneRenderer, didUpdate node: SCNNode, for anchor: ARAnchor) {
        print("Surface updated")

        guard let planeAnchor = anchor as? ARPlaneAnchor else {return}
        node.enumerateChildNodes { (childNode, _) in
            childNode.removeFromParentNode()
        }
        
        let turfNode = createTurf(planeAnchor: planeAnchor)
        node.addChildNode(turfNode)
    }
    
    func renderer(_ renderer: SCNSceneRenderer, didRemove node: SCNNode, for anchor: ARAnchor) {
        print("Surface anchor removed")
        
        node.enumerateChildNodes { (childNode, _) in
            childNode.removeFromParentNode()
        }
    }

}

extension Int {
    var degreesToRadians: Double { return Double(self) * .pi/180 }
}

Made a few edits to update the code and provide better comments