New user, installation process question

Silly question but couldn't find it on your page

When I install your OS, does it delete everything what I currently have on my phone or it moved (files) to the new one.

And if I install new, does my current Android system is getting deleted or just its pushed back and not working.

How I can come back to my previous system if I don't find grapheneOS right system for me?

and 2. "The initial install will be performed by flashing the factory images. This will replace the existing OS installation and wipe all the existing data."