You can use Siri on an iPhone XR, iPhone XS, or iPhone X in two ways: either with the Side button, or through a "Hey Siri" voice command.
On your iPhone, go to Settings > General > About and look at Model Name. If your phone isn't on the compatibility list, it will keep receiving iOS 18 security patches for a while, but you won't see ...
Revive your unresponsive iPhone by forcing it to restart. Additionally, you can try charging the phone or reinstalling iOS.