Here a screenshot from We.Retail on aem6.3 (edit template -> page design)

When using the template editor you can also specify the clientlibs there.Those can then be specified to switch or implement a new design.

get() can be used when you pass in for example a parameter.For the use-cases to get a property you need to access it via getProperties().Where have you seen that in the documentation?

Also WcmUse is bound to your component, while your sling-model can be reuse across your application.

Also the the core-component implementation here: https://github.com/Adobe-Marketing-Cloud/aem-core-wcm-components/tree/master/content/src/content/jcr_root/apps/core/wcm/components/image/v1/imageI'd be careful using the fileReference-property directly, you might reference a large asset to the client.

The one in /apps/core is new in AEM6.3, part of the new core components.The other location is from the foundation components, introduced in 6.0If you are starting a new project/implementation I would use /apps/core

Expressions are done like this:${ pageYouWantToLinkTo @ extension='html'}

Are you sure you have deployed your code into AEM?Check /system/console for when your bundle has been updated

This only applies when you have a page you want to render via data-sly-resource