Why has "WhatsApp" replaced "message" on my iphoneSE

The Messages app can’t be deleted from your phone. So, it’s still there. Have you tried looking for it in the App Library? Have you tried searching for it?

Find your apps in App Library on iPhone - Apple Support

Use Search on your iPhone, iPad, or iPod touch - Apple Support

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

WhatsApp Business is not an Apple product. Contact the app developer here --> Developer Website  or App Support  or WhatsApp business products - WhatsApp FAQ

Can you please provide more detail about the issue.