How could I get the Demo version without from Steam?
The demo can be found on AlterStaff's itch.io page. It's under "Yandere AI Girlfriend Simulator."
The itch.io demo requires an OpenAI API key to function. Which you have to pay for. The developers of the game don't get any money from you buying an API key, by the way.
If you want to try out the game without paying, you need to download the steam version. Or, you can pay for an API key and use that to play the itch.io demo. Or, you pay $10 for this version of the game. If you pay for the full game you can play as much as you want. If you use an API key to run the demo you would pay based on how much you use the API key.