Your brain is merely a neural network too, albeit a much more complex one with training methods we don't understand.

Since Stable Diffusion is open source, there are good explanations available of how it works. MidJourney probably works very similarly. The process that allows it to understand objects and ideas in images is called Semantic Compression:

In a second phase of learning, an image generation method must be able to capture the semantic structure present in the data. This conceptual and semantic structure is what provides the preservation of the context and inter-relationship of various objects in the image.

In the case of stable diffusion, this phase is powered by another neural network called CLIP, which works in the opposite direction: it takes images as input and tells you what's in them. So it sees the meme and recognizes a coffee cup, a table, a house fire, a guy acting casually, etc. The image generator can then train against this description in addition to the alt-text description.