What is "apple.com-online.gifts" that locks Safari on my iPhone?

It is a scam. Ignore any such pop up messages.

Read Phony "tech support" / "ransomware" popups and web pages for a description and solution.

Apparently Nissan's website (like practically every Internet site except for Apple) hosts advertisements over whose content they probably claim they have no control. That creates opportunities for scam artists of all description. It's also possible (though unlikely) that Nissan's website was "hacked". It is common to find such scams while browsing lowbrow sites catering to prurient interests, but I have encountered those scams on a number of allegedly respectable websites. It's simply an Internet annoyance that we have to put up with.

Another possibility is that your Mac may have been affected by adware, another rapidly growing Internet annoyance. Read the following to determine if that may be applicable in your case: How to install adware. Apple maintains a corrective procedure that I linked near the end of that document.

You know, I just went to nissanusa's website and encountered no advertisements... other than Nissan that is.

You probably shouldn't be seeing any either. I suspect adware may be a factor in your case. Read the User Tip I linked.