How to factory reset my macbook pro 2015 version

Since you have never upgraded macOS from 10.11 El Capitan, your options are very limited here. If your laptop is able to boot into macOS or into Safe Mode, then I highly recommend you create at least a bootable macOS 10.14 USB installer (your laptop can run up to macOS 12.x Monterey which is the current version of macOS) using the instructions in this Apple article:

How to create a bootable installer for macOS - Apple Support

If you try to boot into Recovery Mode using Command + Option + R to access the local recovery mode, then you may end up having troubles re-installing macOS because the installer will be outdated which may require changing the system date & time to sometime in 2017.

Upgrading the laptop to macOS 10.13+ gives you a lot more options since it give you access to Internet Recovery Mode to access the online macOS installer.

Hi,

See: What to do before you sell, give away, or trade in your Mac - Apple Support