Make use of custom fonts with the help of extensions. Note: iOS not detecting `black` font weight for Montserrat font. Please comment if you know the solution for it.

Fonts.swift

// MARK: - Usage
func application(_ application: UIApplication, 
didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
  UIFont.registerAllCustomFonts()
  // Or You can add manually,
  // https://developer.apple.com/documentation/uikit/text_display_and_fonts/adding_a_custom_font_to_your_app
  return true
}

// Using Default family name
let customMontserratFont: UIFont = .font(weight: .bold, size: 20)
label.font = customMontserratFont
subLabel.font = customMontserratFont.withWeight(.semiBold)
hintLabel = customMontserratFont.regular

// Or other family
let americalFont: UIFont = .font(.americalTypewriter, weight: .heavy, size: 20)
label.font = americalFont
hintLabel = americalFont.withWeight(.light, fontSize: 18)

// MARK: - Implementation
extension UIFont {
    enum Fonts {
        case americalTypewriter, montserrat

        var family: String {
            switch self {
            case .americalTypewriter:
                return "American Typewriter"
            case .montserrat:
                return "Montserrat"
            }
        }
    }
    
    var ultraLight: UIFont { withWeight(.ultraLight) }
    var thin: UIFont { withWeight(.thin) }
    var light: UIFont { withWeight(.light) }
    var regular: UIFont { withWeight(.regular) }
    var medium: UIFont { withWeight(.medium) }
    var semibold: UIFont { withWeight(.semibold) }
    var bold: UIFont { withWeight(.bold) }
    var heavy: UIFont { withWeight(.heavy) }
    var black: UIFont { withWeight(.black) }
    
    static func registerAllCustomFonts() {
        // Note: iOS not detecting `black` font weight for Montserrat font. So I'm forced to use
        // UIFont(name: "Montserrat-Black", size: 20)
        UIFont.register("Montserrat-Black")
        UIFont.register("Montserrat-Bold")
        UIFont.register("Montserrat-ExtraBold") // UIFont.Weight.heavy
        UIFont.register("Montserrat-SemiBold")
        UIFont.register("Montserrat-Light")
        UIFont.register("Montserrat-ExtraLight") // UIFont.Weight.ultraLight
        UIFont.register("Montserrat-Medium")
        UIFont.register("Montserrat-Regular")
        UIFont.register("Montserrat-Thin")
    }
}

extension UIFont {
    static func register(_ name: String, extension format: String = "ttf") {
        guard let url = Bundle.main.url(forResource: name, withExtension: format) else { return }
        CTFontManagerRegisterFontsForURL(url as CFURL, .process, nil)
    }
    
    static func font(_ font: Fonts = .montserrat, weight: UIFont.Weight, size: CGFloat) -> UIFont {
        let descriptor = UIFontDescriptor.with(family: font.family, weight: weight)
        return UIFont(descriptor: descriptor, size: size)
    }

    func withWeight(_ weight: UIFont.Weight, fontSize: CGFloat? = nil) -> UIFont {
        let descriptor = fontDescriptor.with(family: familyName, weight: weight)
        return UIFont(descriptor: descriptor, size: fontSize ?? pointSize)
    }
}

extension UIFontDescriptor {
    static func with(family: String, weight: UIFont.Weight) -> UIFontDescriptor {
        var attributes = [UIFontDescriptor.AttributeName : Any]()
        var traits = (attributes[.traits] as? [UIFontDescriptor.TraitKey: Any]) ?? [:]

        traits[.weight] = weight
        attributes[.name] = nil
        attributes[.traits] = traits
        attributes[.family] = family
        return UIFontDescriptor(fontAttributes: attributes)
    }
    
    func with(family: String, weight: UIFont.Weight) -> UIFontDescriptor {
        var attributes = fontAttributes
        var traits = (attributes[.traits] as? [UIFontDescriptor.TraitKey: Any]) ?? [:]

        traits[.weight] = weight
        attributes[.name] = nil
        attributes[.traits] = traits
        attributes[.family] = family
        return UIFontDescriptor(fontAttributes: attributes)
    }
}