If you want to read about the theoretical underpinnings, Cassandra is derived from the original Dynamo paper[0] from Amazon, and Scylla is a drop-in replacement for Cassandra written in C++ instead of Java. Cockroach follows more closely the Google Spanner[1] approach.
For a more practical summary, compare the architecture overviews of Cassandra[2] and Cockroach[3].