If I factory reset my phone, will I get its original stock image back?

[Xminent]

I recently rooted, and later un-rooted my LG Stylo 2 Plus using TWRP, installing SuperSU. I found out about this other root called Magisk, that can apparently mask your status of being rooted. I tried my very hardest to install it, but it would not work. It mentioned that one of the requirements was to have a stock image, but I'm pretty sure I downgraded my image to Android 6.0 after flashing another stock rom from online. All of this has got my head wrapped around and I'm not sure if a factory reset will just brick my phone or something. I wanted to know if there's any way I can fix this mess.