Item values displayed in ArticlePlaceholder should be linked to articles if pages exists, otherwise probably to another ArticlePlaceholder page.
External identifier should also be linked.
Description
Description
Related Objects
Related Objects
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External identifiers will be linked by the time we also do so within Wikibase itself, that is going to happen soon. Nothing ArticlePlaceholder specific to do for that (luckily).
The same probably applies to item/ property values, please see T107407.