I've created a smart device with esp32 using adafruit IO and integrated it with google assistant using IFTTT. This way I can control the device using custom voice controls from google assistant or access the adafruit IO panel and toggle the device.
The problem is that I can't find a way to add this device into the google home app, to control it alongside my other smart switch.
Apparently, there is a google home SDK, but I'm not sure how I could integrate my esp32 to it.
You'll want to use Google's smart home platform and build a cloud integration. You may be able to follow Google's codelabs to get started or check out one of the existing code samples.