Smart Plugs you can find online limit you to using Amazon Alexa or Google Home for controlling them. What's worse is they are inextricably linked to the cloud. Unlocked Automation sells high quality Smart Plugs that you can use how you want...
The open source Tasmota firmware supports MQTT as well as HTTP requests as control mechanisms. Cheap smart plugs typically have a smartphone app to setup and control them and the ability to link to Amazon Alexa or Google Home services. While this is great its not helpful if you want to be able to control AC devices programmatically...
Hacking these devices is no guarantee either. The Tuya software they often come with is regularly updated to prevent you from putting open source firmware on them. You can open them and solder wires to the PCB for reprogramming but this can be time consuming and challenging depending on the design of the plug.