GO3D browser viewer uses Three.js 0.180.0 (MIT; see LICENSE) and occt-import-js 0.0.23 / OpenCascade (LGPL-2.1; see OCCT-LICENSE.txt).
Three.js source: https://github.com/mrdoob/three.js/tree/r180
OpenCascade importer source and build instructions: https://github.com/kovacsv/occt-import-js/tree/c2148e54b456b571238d35cac037d304053d64b2
The OpenCascade JS and WASM modules are unmodified and separately loaded. You may replace them with compatible builds. GO3D does not restrict reverse engineering of those components for debugging modifications under their license.