As the title states, I'm sure plenty of people have researched on whether they can build AAA game assets without having to spend any money. In short, the answer is yes, but free does have its challenges. Not saying that it can't be done, because it certainly can, but it might require a few extra steps in order to get the final result.
The first step is finding a modeling and sculpting software (if you don't already have one). Blender is a robust software that allows you to model hard surface and sculpting in order to get your high and low poly models for baking maps. Not too mention that if you are creating characters, you can do your rigging and animations along with it. Another good sculpting software that you can use would be Sculptris, which is essentially a free version of Zbrush. Sculptris would be your high poly model editor where you can add in the fine details.
A side note for Sculptris, you can also use this to unwrap you rUV and do your texturing. Blender has the capabilities to do UV and texturing as well but it all comes down to preference of workflow.
Now that you have your high and low poly model, you will need to bake it all together to get your maps. A nice little free plug-in for Blender to help with that would be TexTools. This little tool will bake your high poly model into your low poly model, and give you the other maps that you will need to really bring your model to AAA quality. You will be able to get your ambient occlusion, curvature, material ID, and normal and more.
Now that you have your model all drawn, your UV all set, and your maps all baked, you can now move on to the next stage of texturing. If you want, you can do all of this in either Blender or Scupltris depending on your preferences, but there is another option. If you want to stick to using as few programs as possible, then check out some YouTube tutorials on how you can streamline your process.
Quixel has a free 3D texturing software called Mixer that has the capability of using and creating smart materials, and that is where all those maps that you baked come into play. Whether you used the TexTools plug-in or used Blender's standard workflow, you should be able to bring those maps into Mixer.
Mixer allows the use of smart materials, which can accelerate your design and give you incredible control of your designs. As an added bonus, Quixel has a file manager tool for free called Bridge, and this lets you see your files in 3D and fully textured. That and it has the capability of exporting to game engines.
Watching some of the example videos along with YouTube has to offer, you can see just how powerful it is and what kind of assets you could create. The only downside to this would be all the steps that it takes to get to this point, as I said before, free has its challenges.
Although there are a few workarounds to get game assets to AAA quality using the 100 percent free path, it still is possible. There are a number of other programs you can buy and use to speed up your process, but that all comes down you what your budget is. You can find something to bake your maps, unwrap your UV's, and even more modeling features.
If you were going to buy something, it would be good practice to research and try to get the most use out of the software. First one that comes to mind is Substance Painter. Substance Painter can unwrap your UV and even bake your models. Not too mention that is has smart material capabilities. This has a monthly subscription or a perpetual license, although not overly expensive, it does cost something in the end. A second option could be 3D Coat. This has all the same features as Substance Painter but with its own sculpting and retopo features, the downside is that it is a little more expensive..
There are quite a few programs out there to speed up your design process, some free, some not so free, but all capable of building AAA assets.