ESP-IDF
Espressif's official development framework for the ESP32 — the thing running underneath even when you think you're writing Arduino code.
The IoT Development Framework arrived with the ESP32 in 2016 and bundles drivers, Wi-Fi and Bluetooth stacks, TCP/IP, OTA updates and flash encryption. At its core is a modified FreeRTOS that can schedule tasks across both cores. The build moved to CMake and the idf.py tool, and configuration happens in menuconfig — the same Kconfig the Linux kernel uses.
The Arduino core for ESP32 is built on top of ESP-IDF, as is the MicroPython port. Anyone who outgrows setup() and loop() ends up here sooner or later, and discovers that Wi-Fi had been running in its own task all along.