Q1. The commissioner needs to know the joiner (sensor) PSKd and Long EUI Address through an out of band mechanisms like scanning a QR Code, user input, keypad etc..
Q2. Threadgroup recommends printing the PSKd and EUI at the end device package i.e. A QR Label may be scanned by a smartphone capable to communicate with a thread border router. Once the joiner credentials are known by a smartphone this can be used as the active commissioner.