The VDB files were created using EmberGen.
According to Jason, the VDBs themselves were created using JangaFX's EmberGen, a real-time fluid simulation tool for creating fire, smoke, and explosions. Additionally, the author used Eidos-Montréal's useful "OpenVDB and NanoVDB for Unreal" plug-in that allows reading OpenVDB and NanoVDB files in Unreal. This plug-in was made specifically for Unreal Engine 5 and can be downloaded here.
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