Apple does not release the macOS Ventura installer as a .dmg download. Here is the only official way to download the macOS 13.x Ventura installer while booted from a Qualifying Mac (that is one which is compatible with Ventura):
How to download macOS - Apple Support
The only macOS installers Apple provides in .dmg downloads are the older macOS installers 10.12 and earlier (or system update patches for macOS 10.15 and earlier).
The only other way macOS Ventura is in a .dmg file is if you downloaded the installer from Apple and manually created a .dmg archive containing the installer.
Otherwise, you downloaded Ventura from an unauthorized third party. This software could be damaged or even worse contain malware. Use at your own risk. Plus you will not get any assistance for using it here on these forums since we have no idea what that file contains.