On one hand, Olah et al.’s (2020) investigations find circuits which implement human-comprehensible functions.
At a higher level, they also find that different branches (when the modularity is enforced already by the architecture) tend to contain different features.
I believe Richard linked to Clusterability in Neural Networks, which has superseded Pruned Neural Networks are Surprisingly Modular.
The same authors also recently published Detecting Modularity in Deep Neural Networks.