The execUsd library is built on top of exec and esfUsd.

The execUsd library is the primary entry point for OpenExec. The API defined here, and in the exec library, supports:

Registration of computational behaviors associated with USD schemas (see the Computation Definition Language documentation)
Ingesting a UsdStage to compile the data flow network that contains nodes that embody computations (see ExecUsdSystem)
Requesting values for efficient, vectorized, multithreaded evaluation (see ExecUsdRequest)

See the OpenExec overview for more details.