You start the ID.3/ID.4 by holding down the brake pedal and then engaging a gear. The gearstick is located on the right behind the steering wheel. Turn the switch from the "Neutral (N)" position either forward to "Drive (D)" or back to "Reverse (R)". Now step on the accelerator as usual to start driving. To park, press "P" on the right side of the gearstick - this is the parking position and activates the handbrake.
Tip: The car saves energy while driving if you put the gearstick lever in "B". This mode charges the battery as soon as you take your foot off the accelerator.
Here you can find a video that explains this in more detail.