Thank you @tmigone, that answers my question. The StackExchange thread says that:
Please note that using the
aplay.servicewhich constantly sends audio to the audio output will break the microphone.
A previous thread (Base image for Raspberry Pi 3 B+, speaker bonnet, I2S microphone and camera - #17 by tmigone) mentioned aplay as a way to avoid the popping sound on the I2S speakers. So I don’t think I can use the I2S microphone, the I2S speaker, and have no popping sound all at the same time.
One solution for the time being is to use the I2S microphone, which records sound in digital format, and use the audio jack for sound output.