Signal app on iPhone 14 Pro after iOS update

No, the app is not part of iOS 18, any version. It had to be downloaded from the App Store. It is a well known encrypted communications app.

There has been so much discussion about malware threats from texting between iPhone and Android phones it makes me not to text anymore and just call people :). Is there any software that can be loaded on iPhone to prevent malware from Android phones?

You're conflating two things: malware and compromised communications. The current media brouhaha is about compromised communication. It is believed that it unencrypted text and voice communications on US carriers may have been compromised. That means that, theorectically, someone could listen to your calls and read your texts. I've seen no concrete evidence that it has actually happed to anyone.

You can't get malware from Android phones. They use a different operating system. Unless you jailbreak your phone, it's almost impossible to get malware.

If you have concerns about your communications, use apps like Signal, iMessage and FaceTime.