Many people are running an open-source re-implementation of the modem [0]. Neither the proprietary nor the open-source firmware for the modem are perfect, but the latter has the advantage of being clocked down to 100MHz instead of the default 400. This allows much better battery life, as well as giving off far less heat :)

[0] https://github.com/Biktorgj/pinephone_modem_sdk

Modem is a heatsink for pretty much all the other heat producing stuff in that phone. It gets hot when whatever else gets hot (PMIC when charging, or SoC when doing soc stuff).