BTE hearing aids got enough space to hold lots of buttons and features. It varies from one device to another since every manufacturer places the things he thinks are most important. It also Varies because BTE comes in many sizes. The bigger the device, the more stuff you can put on it. I mention here the very basic behind the ear hearing aids parts an as I just mentioned, some of the BTEs got additional buttons or functionality. Here are the basics and the most common parts with a few words on the functionality of each part.
Comes in many sizes and shapes. On most cases, you have to have an earmold impression because it is tailor made for your ear. If it's an open-fit, there is no earmold but some tiny tip that replaces it.
Holds the whole thing in place, it is made of a tube that conducts the sounds from the case to the earmold.
The "body" of the behid-the-ear hearing aids and its biggest part. Hosts the "brain" of the device.
It's big enough so you could open it on your own and replace batteries.
Volume control can be either a small wheel or something like two buttons, one for increasing the volume and one for decreasing the volume. On some BTE hearing aids, turning the wheel to its minimum turns off the device.
On most BTE devices (nowadays) you can switch between few programs or modes. They are pre-configured and the device have a small button so you could move from mode to mode without any assistance.
As simple as it sounds... To turn the device on and off.
BTE devices might have more than just one microphone. They use multiple microphones to get more information from the sounds it receives.