Project Fi lists the Pixel, Nexus 5X and Nexus 6P as the only compatible phones with their auto network-switching system, and the FAQ and many reviews state that these phones have special radios:
These devices are the first smartphones that support our network of networks. They work with the Project Fi SIM card, which supports multiple cellular networks, and have a state-of-the-art cellular radio tuned to work across network types.
However, in the iFixit teardowns they don't have any unique hardware. iFixit says that the Nexus devices use the Qualcomm WTR3925 LTE Transceiver, same as several other phones, while the Pixel uses WTR3925 as well as the Qualcomm WTR4905 LTE RF transceiver. All of these radios are used in several other phones, so why don't those devices support carrier aggregation?
I found this Qualcomm article from 2014 talking about carrier aggregation radios and they appear to be widespread:
Can the hardware be enabled in software in other devices?