I design and sell IoT Kits©. These are low-cost, simple kits that can be built with common tools. Most kits use microcontrollers, typically the D1 Mini, ESP32, or Arduino Nano. The hardware and software are open-source. All development software and online services are free for hobbyists and non-commercial users. Assembly and programming of the kits is cookbook style with detailed explanations provided so that you can learn how the devices work. I generally program in the Arduino dialect of C++ so there are many online sources for support and education.
Take a look around the site. I suggest stating with the IoT Kits page. You do not have to register to use the site, however, only registered users can post comments. I will never share your e-mail address with anyone. Occasionally, I may send users notices of new products or updates of kit software. You may opt out at any time.