Where do smaller competing dictionary apps get their definitions from?

No, dictionaries aren’t allowed to copy each other unless they have permission. AIUI however, some older editions of Webster’s dictionaries are in the public domain and so other companies can use the content and call themselves “Webster’s”.

and [according to WSJ investigations] these smaller dictionaries are heavily subsidized by the companies putting serious user tracking software on their sites. I am now avoiding them.

Wiktionary is much like Wikipedia in where they get their content.