How do these video tools work?
Modern browsers include a hardware-accelerated video engine called WebCodecs. ToolsCrow uses it to decode and encode video directly on your phone or computer, so there is no server to upload to and nothing to wait for except the processing itself. Where a video can be changed without re-encoding (removing audio, changing container, lossless cuts), the tools copy the data as it is, which is instant and keeps original quality.
Because everything depends on what your browser can decode, a few old formats (AVI, WMV, MPEG-2) are only partly supported, and the tools tell you plainly when a file cannot be handled.