Square was originally a single RoR monolith. We spent a decade burning it to the ground with a Java and Go microservice architecture.

Some product surface area remains Ruby, but Ruby was chased away by most teams.

Square brought in a lot of Xooglers over the years to lead the transition, so you see a lot of Google tech: protobufs, gRPCs, at one point a pre-Kubernetes Borg clone, etc.