Liehjan , Oct 25, 2020 : You shouldn't have the backup file in the phone storage... Backup to a harddrive or cloud.
KshtizBhattarai , via OnePlus 7 , Oct 25, 2020 : I backup to oneplus offical but its took my extra enternal storage
Liehjan , Oct 25, 2020 : If it takes internal storage, you have the backup file in internal storage... Move the file!
Dr.Abdur_Rahman , via OnePlus 7 , Oct 25, 2020 : Backup occure in cloud it's doesn't use any extra storage
MSD2019 , via OnePlus 7 Mirror Gray , Oct 26, 2020 : Backing up to Lap will store huge space on your device rather then storing on your mobile.
Iamhasannaqvi , via OnePlus 7 , Oct 26, 2020 : Backup is being stored in cloud storage therefore it doesn't occupy internal storage.
Rezmsa , via OnePlus 7 , Oct 28, 2020 : Do backup then save your files through cloud storage or pc prior to factory data reset.