All OpenTitan software is built with Bazel.
For example, to build and run the OpenTitan UART smoke test located in sw/device/tests/ for Verilator, run
cd "$REPO_TOP" bazel test --test_output=streamed //sw/device/tests:uart_smoketest_sim_verilator
The resulting binaries will be located under bazel-out/. For more information, check out the Building Software guide.