The Teensy has two internal ADCs. All analog capable pins can be switched to one of the two ADCs by an internal multiplexer when a command like analogRead is invoked. The Teensyduino core (or, for extended functionality, the ADC library) takes care of that. The hardware allows to either use an internal bandage reference, Vdd, or the AREF input voltage for each of both ADCs independently.
Pins A10 and A11 are somewhat special, since these are “analog only” which means that they can not be used alternatively as digital GPIO pins like A0-A9. And they give the option to be used as either independent analog inputs, or to be “coupled” for differential measurements. But to explore these options, you need the ADC library, the standard functions do not support such special modes.