Is my data safe on a Lost or stolen iPhone?

They say the FBI can’t get into iPhones, yet every travel page warns that thieves or hackers can access my data on my iPhone if I lose it while travelling or if it’s stolen.

Which is it?

I also see two pieces of advice.

Either put your texts on the Lock Screen so I can text my phone number (from another phone) to whoever finds my lost iPhone, or immediately login to iCloud and set my iPhone as lost so no one can get inside it. But you can’t do both.

So, is my phone data safe (I have passcode and Face ID set up).

thanks!