Why does macOS replace .img with .ndif extension? I'm trying to download a custom android recovery image on my Mac but the extension is changed from .img to .ndif automatically. Is this normal? Will it have any impact on the installation on actual image?
From this site explaining what the ndif extension is:

NDIF is an acronym for New Disk Image Format. The .ndif file extension is commonly used for files that contain disk images for computers that are running on the Mac operating system.
An NDIF file contains an image of a disk or a drive that was copied to the file from the user's hard drive. This information can then be used to restore data that has been lost or corrupted.

I can't tell you if that specific file is 'normal' or is safe to open (you can use various unzipping/unraring applications to open it), that's up to you depending on where you got the file.