It’s not coincidence that the app is named GoogolPlex, as what it actually does is intercept Google searches for Plex and redirect them to a webserver that actually does the work. So when you give the command “GoogolPlex Turn on the lights,” Siri interprets that as an instruction to Google “Plex turn on the lights.”
By changing your wifi connection’s proxy settings, these ‘searches’ are redirected to the web app, which then interact with the relevant API to do what you’ve asked of it.
With the approach dependent on the app name sounding like Google, don’t expect it to last long, however …
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