USB or lightning outputs have digital only. They cannot output analog audio. So these phones do not have audio DAC nor amplifier. DAC and amp are on dongles or external devices.
Phones with headphones jack have built in DAC and tiny amplifier, which can output analog signal directly.
As to why OP's external device works with iPhone and not pixel is likely compatibility. Dongle will state which device it is compatible with.
Wow, so many incorrect info on this thread.
USB or lightning outputs have digital only. They cannot output analog audio. So these phones do not have audio DAC nor amplifier. DAC and amp are on dongles or external devices.
Phones with headphones jack have built in DAC and tiny amplifier, which can output analog signal directly.
As to why OP's external device works with iPhone and not pixel is likely compatibility. Dongle will state which device it is compatible with.