Generally you should be able to double click the file you downloaded and it will open up and let you access the installer. Is it possible you changed your system’s configuration in some way?
As far as I know, all macOS computers can open dmg files by default: https://www.howtogeek.com/362166/what-is-a-dmg-file-and-how-do-i-use-one
I'm not entirely sure, and they have been changed, I wouldn't know when or how they were, or how to change them back.
I opened Adobe Flash and Google Chrome the exact same way, and neither of them had any problems. But for some reason, itch just doesn't want to open, even if Google and Flash were the same file types.
I've downloaded the application multiple times, so a single faulty download couldn't be the issue here.
