If a site tells you that you need to download a specific "special player" or "missing codec" to view the .mp4 , it is almost certainly malware. Standard players like VLC or MPC-HC can play almost any legitimate .mp4 without extra software.
Split archives are a relic of an era with strict upload limits, but they remain popular today for bypassing file-size restrictions on free hosting sites. By breaking a large high-definition video into 500MB chunks, uploaders can distribute massive files across platforms that might otherwise reject them.
Suggests this is a newer version of a previously released file, possibly with higher resolution or corrected data. How to Open .7z.001 Files
The final format of the media contained inside the archive.