ESP8266’s technical documents are all listed here:
https://www.espressif.com/en/products/socs/esp8266
PCA9555 is a I2C device, for I2C device, because the data lines are pulled high by the microcontroller, and the slave devices only pull these data lines to ground, it’s ok to have a 3.3V microcontroller interfacing with a PC9555 powered by 5V. 3.3V is within the high logic range of PCA9555, so this works fine.