Hi Steave,
OutSystems SILK UI was used when OutSystems only supported Traditional Web applications. As Mobile (first) and Reactive (later) applications appear and leverage on a different technology (React) and on client side, OutSystems UI was launched to support these scenarios as OutSystems SILK UI doesn't. OutSystems SILK UI is from then deprecated and replaced by OutSystems UI.
If in Traditional Web applications you can still use OutSystems SILK UI (but you should use OutSystems UI Web now since it has more features, work better and OutSystems SILK UI is deprecated) in Mobile and Reactive applications you can't use OutSystems SILK UI, you should instead use the OutSystems UI layouts, themes, templates and patterns.
Instead of using https://silkui.outsystems.com/ as a reference use instead https://outsystemsui.outsystems.com/OutsystemsUiWebsite/
As OutSystems UI was built from the previous OutSystems SILK UI, the way to use is identical so you should not have any problems using the first if you have experienced the latter.
Kind regards,
João