is it easy to install spyware on iphone (not asking for instructions)

Hi I just heard that my mother in-law had installed spyware on her ex's cell phone and it started making me wonder if I could possibly have any installed on my phone. My gf isn't too technically savvy so I'm wondering if it's easy to install spyware on the iphone. I don't want any instructions or steps on how, but wondering more in general. like is it as simple as clicking on a link, is it easy to hide spyware for someone who isn't specifically great with computers but definitely smart enough to follow a tutorial? I've looked through my settings and don't see any apps that are unfamiliar. I don't think my gf would do that, but hearing how normal her mom made it seem made me start wondering...