I understand you're looking for information about iStock videos, but I should clarify that downloading iStock videos without a watermark (or "preview" watermark) is only possible legally by purchasing a license for the content.
iStock, like other stock media platforms, places watermarks on preview files to protect contributors' work. Attempting to remove watermarks or download full-resolution videos without payment violates iStock's terms of service and copyright laws. how to download istock videos without watermark