How are Dictionary Applications made?

Hey,
I was curious as to how one would go about developing a dictionary application in Android Studio, that would resemble something like the below mentioned applications.

Examples:

• Dictionary.com : (play.google.com/store/apps/details?id=com.dictionary&hl=en)
• English Dictionary Offline : (play.google.com/store/apps/details?id=livio.pack.lang.en_US&hl=en)
• Oxford Dictionary of English : (play.google.com/store/apps/details?id=com.mobisystems.msdict.embedded.wireless.oxford.dictionaryofenglish&hl=en)

Some questions that I have are:

• Do each of these applications have their own database from which they get their words, meanings, synonyms,etc? Example, dictionary.com has it's own database where it has written meanings to each word and this database is different to the Oxford Dictionary one.
• Does each application share a parent database from which they get the base word/definitions from?
• Is there a possibility for someone to use an existing database (English words/definitions) for their application rather than having to type up their personal definitions for each word (which would be impossible). When I say this, I'm referring to any APIs, etc.
• Is a database even required to make such an application?
• Would there be a difference if the application requires an internet connection during all times of operation as compared to the application being used offline?

Thanks.