In case it helps anyone, a SpriteKit starter with physics turned on and a couple of sprites demonstrating collision detection

GameScene.swift

import SpriteKit
import GameplayKit

class GameScene: SKScene {

    var box1: SKShapeNode?
    var box2: SKShapeNode?

    override func didMove(to view: SKView) {
        let b1 = SKShapeNode(circleOfRadius: 50.0)
        let b2 = SKShapeNode(rect: CGRect(x: -50, y: -50, width: 100, height: 100))

        b1.fillColor = .yellow
        b2.fillColor = .green

        let p1 = SKPhysicsBody(circleOfRadius: 50.0)
        let p2 = SKPhysicsBody(rectangleOf: CGSize(width: 100, height: 100))

        p1.isDynamic = true
        p2.isDynamic = false

        p1.categoryBitMask = 1
        p1.collisionBitMask = 1
        p1.contactTestBitMask = 1

        p2.categoryBitMask = 1
        p2.collisionBitMask = 1
        p2.contactTestBitMask = 1

        self.addChild(b1)
        self.addChild(b2)

        b1.position = CGPoint(x: 0.0, y: 300.0)
        b1.physicsBody = p1

        b2.position = CGPoint(x: 0.0, y: -250)
        b2.physicsBody = p2

        self.box1 = b1
        self.box2 = b2

        guard let scene = self.scene else {
            preconditionFailure("This should never happen")
        }

        scene.physicsWorld.gravity = CGVector(dx: 0, dy: -1)
        scene.physicsWorld.contactDelegate = self

        let boundary =
            SKPhysicsBody(edgeLoopFrom: CGRect(x: -400, y: -300, width: 800, height: 600))

        boundary.isDynamic = false
        boundary.contactTestBitMask = 0
        boundary.categoryBitMask = 0
        boundary.collisionBitMask = 0

        let boundaryNode = SKNode()
        boundaryNode.physicsBody = boundary

        self.addChild(boundaryNode)
    }
}

extension GameScene: SKPhysicsContactDelegate {
    func didBegin(_ contact: SKPhysicsContact) {
        guard let bodyAPosition = contact.bodyA.node?.position,
            let bodyBPosition = contact.bodyB.node?.position else {
                fatalError("This should never happen")
        }

        let bodyA = contact.bodyA
        let bodyB = contact.bodyB

        if bodyBPosition.x < bodyAPosition.x {
            say(body: bodyB, "left")
            say(body: bodyA, "right")
        } else if bodyBPosition.x > bodyAPosition.x {
            say(body: bodyA, "left")
            say(body: bodyB, "right")
        } else if bodyBPosition.y > bodyAPosition.y {
            say(body: bodyA, "top")
            say(body: bodyB, "bottom")
        } else {
            say(body: bodyA, "bottom")
            say(body: bodyB, "top")
        }
    }
}

extension GameScene {
    func say(body: SKPhysicsBody, _ sayWhat: String) {
        guard let node = body.node else {
            fatalError("Should never happen")
        }

        let label = SKLabelNode(text: sayWhat)
        label.fontSize *= 0.75
        label.fontName = "Courier New"
        label.fontColor = .black
        node.addChild(label)
    }
}