When i factory reset my mobile will i revert back to my original os(marshmallow)?Now my mobile is running on android oreo

I have downloaded one apk from xda developer site (thinking one ui theme).I installed on my device but now i am not able to see it in either the theme list or in app list.Pls help me.(https://forum.xda-developers.com/attachment.php?attachmentid=44154...

Most of people who chat in this app are from India. Samsumg member app is only specific for india or also for asian countries like srilanka, bangla ?

I got december patch along with lock screen stories in j7 2016

When apps are in secure folder will it consume seperate  storage  for that apps space ? I had noticed about 250 mb increase in my internal storage.