Where do I put code that runs when two nodes collide or make contact with each other?

Where do I put code that runs when two nodes collide or make contact with each other?

I've already read the article, "About Collisions and Contacts". That doesn't give me all the information I need.

Here is my code so far. The didBegin callback method isn't firing.

import UIKit
import SpriteKit

class GameScene: SKScene {

    let player = SKSpriteNode(imageNamed: "player")
    
    override func didMove(to view: SKView) {
        
        physicsWorld.contactDelegate = self
        
        player.physicsBody = SKPhysicsBody(rectangleOf: player.size)
        player.physicsBody?.isDynamic = false
        player.physicsBody?.categoryBitMask = 0b00001
        player.physicsBody?.collisionBitMask = 0b00001
        
        player.position = CGPoint(x: 20 + player.size.width/2, y: view.frame.height / 2)
        
        addChild(player)
        
        let carEngineStart = SKAudioNode(fileNamed: "car_engine_running")
        
        addChild(carEngineStart)
        
        run(SKAction.repeatForever(
            SKAction.sequence([
                SKAction.run(addCompetitor),
                SKAction.wait(forDuration: 3.0)
                ])
        ))

    }
    
    override func update(_ currentTime: TimeInterval) {
        
        let internalRollSign = TrialSpriteKit.sign(internalRoll)
        
        switch internalRollSign {
        case .zero:
            print("0")
        case .positive:
            print("+")
            if player.position.y < self.size.height {
                player.position.y += 5
            }
        case .negative:
            print("-")
            if player.position.y > 0 {
                player.position.y -= 5
            }
        }
        
    }
    
    func random() -> CGFloat {
        return CGFloat(Float(arc4random()) / /* 0xFFFFFFFF */ 4294967296)
    }
    
    func random(min: CGFloat, max: CGFloat) -> CGFloat {
        return random() * (max - min) + min
    }
    
    func addCompetitor() {
        
        // Create sprite
        let car = SKSpriteNode(imageNamed: "utili")
        
        car.physicsBody = SKPhysicsBody(rectangleOf: car.size) // 1
        car.physicsBody?.isDynamic = false
        car.physicsBody?.categoryBitMask = 0b00001
        car.physicsBody?.collisionBitMask = 0b00001
        
        // Determine where to spawn the car along the Y axis
        let actualY = random(min: car.size.height/2, max: size.height - car.size.height/2)
        
        // Position the car slightly off-screen along the right edge,
        // and along a random position along the Y axis as calculated above
        car.position = CGPoint(x: size.width + car.size.width/2, y: actualY)
        
        // Add the car to the scene
        addChild(car)
        
        // Determine speed of the car
        let actualDuration = random(min: CGFloat(2.0), max: CGFloat(4.0))
        
        // Create the actions
        let actionMove = SKAction.move(to: CGPoint(x: -car.size.width/2, y: actualY), duration: TimeInterval(actualDuration))
        let actionMoveDone = SKAction.removeFromParent()
        car.run(SKAction.sequence([actionMove, actionMoveDone]))
    }

}

extension GameScene: SKPhysicsContactDelegate {
    
    func didBegin(_ contact: SKPhysicsContact) {
        
        print("BANG!")
        
    }
    
}