I bought some wifi enabled color changing "smart" lights that work with the Google Home app and Amazon Alexa but not with Apple HomeKit. As someone who's heavily invested in the Apple ecosystem, this came as a disappointment and it made me wonder if it's possible for manufacturers to make their devices compatible simply by pushing a firmware update or if there are specific hardware requirements that have to be met.
Manufacturers can add HomeKit support through post-purchase updates but not many have as there are licensing and customer support costs associated with integrating with another smart home ecosystem.
You should look into HomeBridge. It’s a service that runs on a home server (iMac or Mac mini work great) and “bridges” the gap between non-HomeKit devices and HomeKit. For example, I have it setup and am able to control my Nest thermostat (temp and fan) from the Home app.