Assuming "Interwiki" means between wikis, you can quickly link to Wikipedia entries by prepending
Wikipedia: to the article name in Wikipedia you want to link to using the same notation as intrawiki links (links within the wiki).
All Wikia wikis can be linked to via interwiki linking between each other. If a Wikia wiki domain is in the form of subdomain.wikia.com, you can link to them using w:c:subdomain:. This is also true of other language versions of Wikia wikis: langcode.subdomain.wikia.com → w:c:langcode.subdomain: