For example, there are scannable images/codes that create augmented reality experiences. I've seen this applied to advertising materials and even T-shirts. If I wanted to apply this to a product it hasn't yet been applied to, can this be patented?
Like many things, the answer is maybe. To be patentable, an invention must be novel, non-obvious and useful. Novel means no one has already thought of it (and published the idea). Non-obvious means that someone skilled in the associated area of technology wouldn't find the idea to be obvious to do. In other words there needs to be some inventive step. Lastly, the invention needs to fulfill some use, it can't be abstract. In this case, if you are taking an idea that exists and applying it to a new product the question is whether there is some unexpected or surprising result of this application. If so, there there is every possibility of patentability. Here is an example. Lets say someone invented silk screening as a method of applying graphics to clothing. If you want to apply silk screening to to canvas bags, I'd guess it would be considered obvious and thus non-patentable. On the other hand if you were the first person to use silk screening to apply conductive inks to fabricate electrical circuits, it probably would be patentable.
An important consideration is freedom-to-operate. You really need to try to track down the relevant patents that already exist. It is quite possible that the claims in those patents are broader than is exhibited in the products you see on market. If so, even if you could get a patent for your specific application, you might need to obtain a license from the original patent holder for the more general use of the technology.
My experience is that it's very unlikely
It sounds like you're asking whether it's possible to win a patent when the invention takes existing technology and applies it to a new product. If I understand correctly, you're suggesting that there's no technical hurdle or obstacle involved in the new application. That is, if you described your idea to a reasonably skilled software developer, he/she could implement your idea without encountering any problems or hiccups.
My personal experience with the scenario I've described is that a patent is not within reach. When patent examiners reject an application, they can draw on multiple different sources to arrive at your invention. So if the examiner finds any patent, article, etc. that suggests using the existing technology in your domain, then the examiner will probably use this to reject your application. Overcoming those rejections can be extremely difficult. I've even seen scenarios where an examiner even rejected an application on the basis of a very broad statement along the lines of "this technology can be used in many other contexts." If the examiner thinks it would've been obvious for an ordinarily-skilled developer, technician, engineer, etc. to do what you've done, then your application will probably be rejected for being obvious or non-inventive.
That said, there are scenarios where applying an existing technology to a new domain can result in a patentable invention. As one example, your application might really involve an inventive step if the literature/state of the art contains teachings that would discourage people from applying the software in the way you have. As another example, applying the software might result in several significant hurdles that your engineers had to overcome. Overcoming those obstacles can result in inventive solutions that rise to the level of a patent.
I am no expert, and this response is for informational purposes only and not for the purpose of providing legal advice. You should contact your attorney or legal expert to obtain advice with respect to any particular issue or problem. Laws can differ dramatically from country to country, state to state, and technology field to technology field. And I might simply be wrong.