I’ve recently been looking into ESP32 programming - they’re microcontrollers with onboard Bluetooth and WiFi, that are smaller yet more powerful than Arduinos. Randomnerdtutorials gets recommended a lot elsewhere; I believe I saw one tutorial for running a web server on an ESP32.

If you need a full OS and/or more resources, I’m not sure raspberry pi can be beaten (at least, that’s how the market was years ago when I was looking)