Out of the box support is limited to the skills offered by manufacturers of smart home devices. But there is a lot of help on the open source community to extend Alexa’s usefulness to devices not currently supported.
This screenshot shows the browser user interface for a server application called HA-Bridge, which runs a Java application that emulates a Philips Hue Bridge. The code is available on GitHub.
The GitHub documentation provided by the developer gives extensive instructions on how to set up HA-Bridge on a Raspberry Pi.
This image shows six devices I can switch on and off and “dim” using commands Alexa normally associates with Philips Hue lighting. So I can ask: “Alexa, switch on television,” and it sends the appropriate sequence of commands to my Logitech Harmony Hub to power-on the TV.