I need to know the exact name of below typography technique or the phrase designers use to mention these kinds of artworks.
Is there any app or online service to make such images?
Those are usually mean word clouds.
You can use this site easily for creating a word cloud.
The term I think you're looking for is 'tag cloud', or 'word cloud'. The average tag cloud doesn't regularly include custom shapes like the arrow on the lower left of your example, but the term is close enough.
It seems you can create great word clouds with Wordle.