Start typing your questions to Siri by following these steps

With the recent release of iOS 18.1, newer iPhone models that support Apple Intelligence's suite of AI tools, are able to utilize a feature called Type to Siri. When enabled, this setting allows you to type out your search queries directly to Siri, as opposed to having to rely on vocal commands.

Source : https://www.pocket-lint.com/how-to-use-type-to-siri-on-ios/