An architecture where the whole backend is one deployable application. Modules call each other directly in the same process rather than over the network.
A well-structured monolith is simpler, cheaper, and faster to develop than distributed alternatives, and it is the correct starting point for the overwhelming majority of products. Monoliths get a bad name from poorly organised ones, not from the model itself.
We build modular monoliths with clear internal boundaries, so that if one part later needs to scale independently it can be extracted without untangling the rest. We do not split a system into services before there is a reason to.