Meaning of network interface rmnet_ipa0

rmnet_data0 is created for USB tethering and for cellular connection. Note that the IPA is an" IP Acceleration" feature from Qualcomm. SO when rmnet_ipa0 exists, it mean you are using a phone with Qualcomm's SoC, and this interface is created for IPA feature. This interface just like another tunnel for cellular data, but without bothering SoC, which reduces the effort of Soc.

rmnet_ipa0 will be there even when your phone is in airplane mode (because setting airplane mode just disconnect modem in the phone from the base-station, but the interface between modem and PC is still exist).

To expand on previous answer... SoC stands for "System on Chip". More than just your device's CPU, it is your device. Save the peripherals (CMOS/camera sensor, display, speakers, mic, etc.), the SoC is literally the entire system on a board (usually not on a single silicon chip, although the CPU and GPU are often), it contains the GPU, CPU, IPC (image processor), WIFI modem, Cellular modem(s), CMOS, data processor for camera, and more, depending on device, like specialized VR processing units, GPU independent varying precision arithmetic units, AI acceleration units, etc.). Think of the SoC as an all in one motherboard on a PC- CPU, GPU, most peripheral (akin to PCI(e) cards) devices are pre-integrated into the chip in order to reduce size, improve electrical efficiency and ease mass production flow.

As for IPA, an IP accelerator would be a physical interface (a special processing unit) on the SoC, so it would not put less strain on the SoC per sé, but rather use a dedicated chip on the system board to handle IP traffic. So the load would not need be burdened by the CPU. This is similar to "hardware offload" on desktop/laptop modems (WiFi, ethernet, etc.) cards. Except in the latter case, the specialized processor for handling then would be a part of the modem card, not on the system.

Note that a net effect would be the SoC doing less work, as the IPA is more efficient at IP packet processing than the CPU unit, since it is specialized for the task.