How to apply a bottom shadow to a given UIView, like for a keyboard key?

If you inspect the keyboard view using Debug View Hierarchy, you can see that the "key buttons" do not have a shadow ... they have a single-point line on the bottom.

Various ways to do that, but one of the easiest is to add a white CAShapeLayer, give both the view's layer and the shape layer the same size and corner radius, and then shift the shape layer up by one point.

Here's a quick, simple view subclass - marked as @IBDesignable so you can see it in Storyboard / Interface Builder:

@IBDesignable
class BottomShadowView: UIView {
    
    let topLayer = CAShapeLayer()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() {
        
        // add the "white" layer
        layer.addSublayer(topLayer)

        // give both layers the same corner radius
        layer.cornerRadius = 8
        topLayer.cornerRadius = 8

        // set top layer to white
        topLayer.backgroundColor = UIColor.white.cgColor
        
        // if background color is not set, or is clear
        //  set it to dark gray
        if self.backgroundColor == nil || self.backgroundColor == .clear {
            self.backgroundColor = .darkGray
        }
        
    }
    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        // if background color is not set, or is clear
        //  set it to dark gray
        if self.backgroundColor == nil || self.backgroundColor == .clear {
            self.backgroundColor = .darkGray
        }
    }
    
    override func layoutSubviews() {
        super.layoutSubviews()
        // make top layer the same size as bounds
        //  offset up 1-point
        topLayer.frame = bounds.offsetBy(dx: 0, dy: -1.0)
    }
    
}

It looks like this (on a systemYellow background):

and zoomed-in for clarity:

We can also get a slightly "lighter weight" custom view by overriding draw() instead of adding a sublayer:

@IBDesignable
class DrawBottomShadowView: UIView {
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }
    func commonInit() {
        // background color needs to be .clear
        self.backgroundColor = .clear
    }
    override func prepareForInterfaceBuilder() {
        super.prepareForInterfaceBuilder()
        self.backgroundColor = .clear
    }
    
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        
        var r: CGRect!
        var pth: UIBezierPath!
        
        // if rounded rect for "bottom shadow line"
        //  goes all the way to the top, we'll get
        //  anti-alias artifacts at the top corners
        // so, we'll make it slightly smaller
        r = bounds.insetBy(dx: 0, dy: 2).offsetBy(dx: 0, dy: 2)
        
        pth = UIBezierPath(roundedRect: r, cornerRadius: 8)
        
        UIColor.darkGray.setFill()
        pth.fill()
        
        // "filled" rounded rect should be
        //  1-point shorter than height
        r = bounds
        r.size.height -= 1.0
        
        pth = UIBezierPath(roundedRect: r, cornerRadius: 8)

        UIColor.white.setFill()
        pth.fill()
        
    }
    
}

I think you can just use the backing CALayer that comes with every UIView for this.

    func keyLetterButtonView ( havingLetter letter : String) -> UIView
    {
        let v = UIView()
        let l = v.layer
        l.cornerRadius = 4.0
        l.shadowOpacity = 1.0
        l.shadowRadius = 0
        l.shadowColor = UIColor.systemGray.cgColor
        l.shadowOffset = CGSize ( width: 0, height: 1.0)
        v.backgroundColor = .systemGray6
        
        // Key letter
        let label = UILabel()
        label.font = UIFont.preferredFont(forTextStyle: .title3)
        label.textAlignment = .center
        label.text = letter
        v.addSubview ( label)
        
        var constraints = [NSLayoutConstraint]()
        for vv in [ v, label] {
            vv.translatesAutoresizingMaskIntoConstraints = false
            let sizeConstraints = [
                vv.widthAnchor.constraint (
                    equalToConstant: 28.0
                )
              , vv.heightAnchor.constraint (
                    equalToConstant: 42.0
                )
            ]
            constraints.append (
                contentsOf: sizeConstraints
            )
        }
        NSLayoutConstraint.activate ( constraints)
        
        return v
    }