Devices you add in Homey will apper in Alexa via the skill and in HomeKit via the Experimental feature in Homey. Not all devices are supported by Alexa and HomeKit though, but most are.
As an example I have created flows for my “Cave” that are trigered by virtual switches, those switches are added in both Alexa (via the Homey skill) and in HomeKit (via the experimental feature by Homey).
I can for example say “Alexa, turn on Playstation” and my Tv, AVR etc. are turned on and switch to right channel (Harmony app in Homey). I can also ask Siri to turn on Playstation and Siri will do the same thing.
Another example is that I can control my Z-wave Danalock (not the HomeKit version) via Siri thanks to Homeys HomeKit function. However the Homey skill in Alexa do not add my lock into Alexa so if I want to control my lock via Alexa, I have to create a flow triggered by a virtual switch that are added into Alexa.
I do not know why the Homey skill in Alexa dosnt add all devices and why the flows isnt added as scenes in Alexa but I hope they will support this in upcoming updates. Home Assistant is much better in that aspect. More device types and scenes are added by their skill.
I dont think any devices added in Alexa or HomeKit are included in Homey. AFAIK no devices are added into Homey from other sources, you have to add them in Homey and then Homey can share those to other services like Alexa, Google and HomeKit.
This is my understanding of Homey works and it was the same thing when I was using Home Assistant before switching to Homey.
This probobly makes you more confused but just keep asking and someone that actually know something will step in.