It depends what they're using. Often, the cross platform libraries are abstractions that just call the native functions for that platform. Even in that case it's difficult because differences to that platform can cause minor layout issues.
There are cases where a custom, bespoke application for each platform is warranted, but that is rare. It's a continuum between the custom, bespoke and a UI/color scheme completely custom to your app.
There are cases where a custom, bespoke application for each platform is warranted, but that is rare. It's a continuum between the custom, bespoke and a UI/color scheme completely custom to your app.