If your phone has virus or not

how do you tell if your phone has a virus ? Can you tell without downloading and paying for unnecessary virus scanner apps ? I accidentally clicked on one of those pop ups that came up in safari that say they are your settings . I clicked on the link but did not download the app it took me too as I had learnt about this stuff in school . I searched if this popup was real and found out it was not . I’m not sure if my phone has a virus or not . I was also wondering that if my phone had a virus and I took it in to apple to get it sorted out what they would do to remove it .

Also if my phone did have a virus , why is that bad.