What happens when you Reset your iPhone to Factory

You lose everything on the phone that you haven’t backed up if you do this→Restore your iOS device to factory settings - Apple Support

However, if you just force-restart the phone→If your iPhone won't turn on or is frozen you won't lose any data

👉 Make sure to back up your device so that you can restore your data later.

See -> How to factory reset your iPhone, iPad, or iPod touch

How to factory reset your iPhone, iPad, or iPod touch - Apple Support

"A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS or iPod software." This uses a computer.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

This support article outlines the various ways you can back up your iPhone, iPad, and iPod touch --> How to back up your iPhone, iPad, and iPod touch - Apple Support

then:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

The ONLY way your iPhone content can be compromised is if someone knows your Apple ID and password and has access to your Apple ID account. Here is guidance on how to check for that→Checklist: If you want to see if anyone else has access to your device or accounts - Apple Support

However, your Facebook is also at risk if anyone knows your Facebook password; that is the more likely source for compromised data.