Macintosh HD file access?

Hi! I took my old MacBook in for service because I wanted to get the files stored on it. It wouldn't turn on anymore. I gave the service an external hard drive and wanted to save the files there. They agreed that they would only copy the user folder from the old Mac, not any applications. I now have a file on the external hard drive called macintoshhd.dmg. I'm familiar with the format from software found online. What's the best way to open the file? I'm really only interested in documents, images, Photoshop files, etc.!

I'm a little worried that if I just open it, it'll try to install something and overwrite my hard drive from the new Mac or something haha.

Thanks !!