SP32 is a series of low-cost, low-power system on a chip microcontrollers with integrated Wi-Fi and dual-mode Bluetooth (https://en.wikipedia.org/wiki/ESP32)

Getting started guide: https://esphome.io/index.html

Tutorial: https://randomnerdtutorials.com/esp-now-esp32-arduino-ide/

Mailbox project: https://smarthjemmet.dk/2021/12/mailbox-sensor-der-holder-stroem-i-12-aar/