Q: If iCloud back up is incomplete when my iPhone loses its Wi-Fi connection does it mean I have to start from the beginning to back up my iPhone or does it continue from where it was the day before.
A: Under normal circumstances, an interrupted iCloud backup will resume from where it last left off, saving you the time required to start over from scratch.
In fact, all backups to iCloud are incremental in nature. Once the initial iCloud backup of your device has been completed, subsequent backups only copy files that have actually changed, making them (usually) run much more quickly than the initial backup.
Keep in mind, however, that partial backup data will not be accessible until the complete backup has finished. If a partial backup has been performed, this is saved on Apple’s servers for a certain period of time to allow the backup to resume later from where it left off, but there is no way to actually get access to it.
Note as well that partial files aren’t saved, so iOS will start over at the beginning of any file that was being backed up when the connection failed. In the case of particularly large files, this means that you won’t necessarily save much time on the subsequent backup.
There are also other reasons why iOS may need to make a fresh backup to iCloud, such as if a lot of time has passed since the last partial backup, or if the backup fails due to some other reason such as file corruption. In this case, iOS will quietly start a new backup, which will take longer to run, however once the new backup is made, subsequent backups should go back to being incremental, only backing up changed files.