If I were to design a UI library in C# for the Unity engine... just a code sketch

example.cs

public static class BackButton {
    
    public static Component = new ComponentDelegate(BackButton.Render);
    
    public struct Props {
        public String Label;
        public Integer Padding;
    }
    
    public static Render(BackButton.Props props) {
        var handleClick = UseClickHandler();
            
        return new[] {
            Render(Button.Component, new Button.Props { OnClick = handleClick, ID = "MyButton" }, new[] {
                Render(Icon.Component, new Icon.Props { Glyph = "BackArrow" }),
                Render(Spacer.Component, new Spacer.Props { Size = props.Padding }),
                Render(Text.Component, null, new[] {
                    props.Label
                })
            })
        }
    }
}

//////////////////////////////////////////////////////////////////////////////

public static class Root {
    
    public struct Props {}
    
    public static Component = new Delegate(Root.Render);
    
    public static Render(Root.Props props) {
        return new[] {
            Render(BackButton.Component, new BackButton.Props { Label = "Backwards", Padding = 16 })
        }
    }
}

//////////////////////////////////////////////////////////////////////////////

Render(Root.Element)