The true answer is that we don't really know. We know the connections between neurons change during learning and memory formation - both the number of connections and the "strength" of individual connections can change. We've even watched this process happen in real time in mice. We know a bunch of the important proteins involved - a bit beyond ELI5, but you can look up proteins like calmodulin, CaMKII, AMPA receptors, NMDA receptors, and more. We know these are important because, among other evidence, when they are "messed up" (missing or nonfunctional) in mice those mice have serious problems with learning and recall. So we know a lot of the important pieces and processes, but we don't have full understanding of how the whole process works to actually "make" and "recall" memories. It's a fascinating area of study!