How can I make my iPhone more safe?

Nothing is ever completely safe or completely secure. Nor can it be, really.

Different people have different security risks, different requirements, and different adversaries, too.

To learn about Apple security: Apple Platform Security - Apple Support

And for your own safety and security: Personal Safety User Guide - Apple Support

To the references already provided, I’ll add:

Use the built-in privacy and security protections of iPhone – Apple Support (AU)

And as Apple devices and systems have become ever more secure; criminals are now focusing more and more on the weakest link … the human user.

So read and heed this:

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Well, the FBI has a really tough time trying to get into an iPhone, not to mention the cost....the going price is around a million dollars.....so unless you have wealth in the billions, you are a high government official or a spy for another country I would not be too concerned about the security of your iPhone as long as you keep the phone updated.