Where is the Type to Siri button in iPhone 16 running iOS 18.4.1?

Have you enabled the feature?

Type instead of speaking to Siri

1. Go to Settings  > Accessibility > Siri, then turn on Type to Siri.
2. To type a request, activate Siri, then use the keyboard and text field to ask a question or make a request.

This is the link to Apple's guide to Siri:

Use Siri on iPhone - Apple Support

It doesn't have ads.

In the settings search bar, you can type Siri and go through her settings.

https://www.siriuserguide.com/how-to-use-type-to-siri-on-the-iphone-and-ipad/

if you turned on the apple intelligence Siri then you should be able to hold down the right button ( off ) and Siri will appear.

good luck