Does factory reset affect Android version?

Yes, a factory reset does not upgrade or downgrade the Android software. A phone operating on KitKat will continue to do so until you perform a software update.

Can I downgrade my Android version?

Things You’ll Need Before You Can Downgrade

Once you’ve backed up all of your data, you’ll need to download an Android factory image of the version you want to go back to, and it has to be specifically designed for your device. Google provides Pixel users with a list of factory images.