Is it dangerous to open the URL in a strange SMS without downloading the app?

It should be no problem to open the website. If it is toxic, as long as a mobile housekeeper or mobile assistant is installed, there will be a risk of prompting. Some mobile phones have their own security monitoring. If you don't click download, it should be no problem. Some are just normal APP promotion. Even if you download it, it's no problem. I suggest you don't screw it up.