Given this paragraph about Twitter, how can programmers connect to Twitter when building custom applications?

Twitter is an online social media and social networking service owned and operated by American company Twitter, Inc., on which users post or reply to texts, images and videos known as "tweets".[9] Registered users can tweet, like, "retweet" tweets and direct message (DM), while unregistered users only have the ability to view public tweets. Users interact with Twitter through browser or mobile frontend software, or programmatically via its APIs.

Programmers can connect to Twitter when building custom applications by using Twitter APIs.