There's several different ways but they all border on similar. But the general way to do it is to usually have program do it for you.

picture attachments via sms / txt msg. its a link with code in it. You receive a text. Move from text list to text msg, you activate and authorize it to run. Just like the email attachment method (beware small file attachments with funny file extensions).

porn sites have the similar exploit but the code is in the video or overlayed it so to speak.

as usual, someone clicks something or opens something with unintended consequences for themselves. sometimes the exploit is undirected meaning its meant for anyone, they'll use a dialer that dials/texts out an "attachment" to a spread of phone numbers. other times its directed.

if you want to read and get you geek on, go to grc dot com and click the research tab, historical, worm wars. or start searching up phone security.