This can depend on the phone. With Android devices, when you connect a USB cable to a PC or laptop, you should see a message confirming taht it's been connected but you then have to accept the USB file transfer facility and when you've done that, you have to OK to accept it again. Connect up and look at the top of the screen for the USB icon then drag that downwards to open the screen where you accept the facility. Only then can you "see" the device on the PC under My Computer.
When you've finished, be careful to make sure you right clicked the device in My Computer then select Eject before unplugging. It's the mobile phone version of "Safely remove hardware". I can't help you with iAnything but that's the process for Android mobiles.