It’s far more than “just a phone” to the point that making calls is one of the least-used features now. And yes, using all that functionality crammed into “the size of a Cliff Notes cheater” does require a different/new user interface.
The iPhone 6s has about the same power as a MacBook notebook computer, same processing speed and same onboard storage. As such, the mere pocket “phone” has expanded to web browser, game console, video phone, and a thousand other uses including handling “enterprise data”. My business lets users access sophisticated corporate data from a range of mobile devices, getting “real work” done from inventory management to medical bedside prescription ordering to on-site travel logging to ... .
Input has been adapted to extensive use of direct touch (so no need for a mouse), touch-based on-screen keyboard (no need for a physical keyboard), and (for you, possibly mind-blowing) “swipe” style keyboard input where rather than deliberately touching each key just sliding your finger from one to another (yes, touching other keys in the process) and they computer figures out what word you were most likely trying to type. Also voice input has grown tremendously, so you can literally talk to the thing.
And yes, these things are so much fun to use that people will delightedly stand in line for hours (or days!) waiting to buy the first of new models. I had the joy of getting the original iPad just 30 minutes after it was released, and now programming iPads & iPhones is my career. Imagine a “car nut” from years past getting all excited about the latest muscle car about to be released from their favorite manufacturer; same idea now, with “mental muscle computers” being released with new & exciting capabilities.
/8^)