Raspberry pi 3 MIPI signal question
The MIPI standard specifies two level of voltages:

One is I2C compatible and it is used to control the camera, read and write the internal registers. This is called the LS (Low speed) state.

The other is the HS state in which the signals work as a high speed differential signal. That is when the data is transported. That can go up to the GHZ range.  

For all the details, e.g how the signals indicate a transfer from one state to the other, you should try to get you hands on the MIPI spec. But as many standards these day you probably have to pay to be part of a consortium to be allowed to have one.