The unit has two e-Tube ports, allowing it to be placed between the rear derailleur and junction B. It is meant to be installed on the seat stay, close to the rear derailleur.
BT-DN110 / BM-DN100 required
The EW-WU111 / EW-WU101 wireless units are meant to be used with the BT-DN110 / BM-DN100 battery/mount and will not work with the older SM-BTR2, SM-BMR1 or SM-BMR2 batteries. The older SM-EWW01 will work with the older batteries, but only supports ANT+ (no BluetoothLE).
E-Tube app
In order to connect the Wireless Unit to your mobile phone you will need either the E-Tube Project mobile app or the E-Tube RIDE app. The latter is meant to be used on E-Bikes, but it'll work on up-to-date road bikes too. Note that if you have never updated your Wireless Unit then it will only work with E-Tube Project.
Pairing a Garmin is relatively simple. The video below shows the process on a Garmin EDGE 830.
The same can be done on a Wahoo ELEMNT device:
And of course, plenty of other bike computers: Sigma, Lezyne, Bryton, Mio, and others all support Di2. You generally add the Di2 system through the sensors menu / page.