In almost all cases Alexa/Google Assistant support works in one of two ways:
Alexa device → Alexa cloud service → Device’s cloud service → Device: Alexa is not communicating directly with your device. It’s going through the Device’s cloud service. And there is pretty strong authorization and authentication between the two cloud services.
Alexa device → Alexa cloud service (to understand the command) → Radio/plug-in on Alexa Device → Device: Alexa is directly communicating with the device, but that is usually only for devices that use technologies like Zigbee or Bluetooth or other non-WiFi wireless technologies.
Given the above:
This is not how it works. And because of the strong cloud-2-cloud authentication you can’t pretend to be Alexa when sending these commands. You need to be independently authenticated and authorized to make the commands to the device maker’s cloud service.
So your real options are to continue to use Alexa or to cut Alexa out of the picture entirely and integrate the device directly into openHAB.
Yes if one of the following is true:
the device maker provides a published cloud API which openHAB can communicate with directly
someone has reverse engineered the device maker’s cloud API
the device is wifi and the device maker has published a local control API which openHAB can communicate with directly (this is very rare)
the device is wifi and someone has reverse engineered the local control API
the device can be reflashed with open source firmware that enables local control
the device uses a standard non-wifi wireless protocol for communication (Zigbee, Zwave, etc.) and you add a USB dongle to your openHAB server that allows for that type of communication
If any one of the above is true, you can integrate the device with OH directly without Alexa.
Here is how that works. The device connects to the device’s cloud service. It’s an outgoing request initiated by the device to the cloud server so your firewall will allow it. With the connection established, the cloud service will forward any commands to the device and receive any sensor readings from the device through that connection.
Anything that wants to interact with the device wirelessly, be it the device service’s web page, the phone app, Alexa, Google Assitant, etc. all must communicate with the device through the device’s cloud server. The cloud server forwards the commands to the device.
Unless the company providing the devices is really bad at security (in which case unplug these devices now, they are not safe to use) all the above communication will be point-to-point encrypted using TLS just like your browser does when using HTTPS. So sniffing the network traffic probably won’t tell you much.
tl;dr: What you are asking for is out of a misunderstanding on how this stuff all works. If you don’t want to use Alexa, you can’t pretend to be Alexa to control devices that are not already supported by OH. You will have to integrate those devices with OH directly, or use the real Alexa service. There are not short cuts and if there were, the security provided is so weak you should not use these devices in the first place.