The Google Pixel 3 XL supports charging using standard USB Power Delivery 2.0 technology but the phone also seem to work fine with Qualcomm Quick Charge 4+ based charging as seen below.
Using the Belkin 27W QC4+ Power Adapter, we plugged in the Google Pixel 3 XL to the Qualcomm Quick Charge 4+ enabled charger supporting USB Power Delivery 3.0 based PPS (Programmable Power Supply) and Augmented PDO's over the USB Type-C interface. Charging was observed to complete in about 123 minutes through USB Power Delivery starting from a fully depleted battery on the phone.
At initial plugin, the voltage immediately started up at about 5 volts while the phone began pulling current below 1 amps. As the voltage rose to near 9 volts, the current pulled progressively increased to reach around 2 amps for about 18 watts (9 volts @ 2 amps) of charging. The current then subsequently bounced up and down continuously between 1.5 amps and 2 amps after the first 10 minutes of charging while the voltage stayed around a 9 volts rail. In the next 40 minutes the current started reducing to about 1.4 amps for the phone to charge at around 12.5 watts (8.9 volts @ 1.4 amps). The current then gradually decreased when halfway through charging and later dropped further down to about 0.1 amps when reaching full charge on the battery. Click here to view more details by downloading the trace captured by the GRL-A1 power analyzer.