How to scan for malware or spyware on iPhone after accidentally opening a link?

Opening a link by mistake is harmless.

Now.....if you clicked to open the link....and....also.... clicked to give permission for an app to be installed on your iPhone.....that is another matter.

It is extremely unlikely that malware was installed on your iPhone. For example, you would see multiple warnings even before you could proceed.

Apple has the iOS system so tightly secured that I suspect you would have to try very hard and intentionally to get those on your iPhone. Read about how to use the built-in security and privacy protections of iPhone in this article--> Control what you share on iPhone - Apple Support