There are a ton of image manipulation options out there. Free would be inkscape and gimp. I’m not proficient or even mediocre at either and perhaps one would be good enough, but I’ve used both for various production elements I’ve created or modified. I don’t use iPhone, so I’ve got nothing for you there. I know that Pixel phones have built in editing tools, so I assume there’s an iPhone option that will more or less hold your hand through some level of manipulation.

The knowledge behind the fundamentals of editing images is fairly equivalent across platforms. Things like layers and some common tools exist everywhere, it’s just knowing which button to use. There will assuredly be resources out there for what you want. Think of specifically what you want to do, and search for solutions…when you come across a solution that mentions something that you aren’t familiar with, open a new tab and dive down the rabbit hole. Eventually, things will start to click and your ability to absorb new information will go much quicker since you have built a solid foundation.