FLV
FLV is the Flash Video container that carried almost all web video from 2005 until Flash was retired in 2020. Browsers can no longer play it, but FLV files still turn up in old course downloads, archived webinars, and legacy CMS exports. Convert FLV to MP4 to make that footage playable again on any modern device.
MP4
MP4 (MPEG-4 Part 14) is the most widely supported video container on the planet. It packs H.264 or H.265 video with AAC audio into a single file that plays natively on iPhone, Android, Windows, macOS, web browsers, smart TVs, and every modern editor. Use MP4 when you need a finished, shareable video that will just work, anywhere, without re-encoding.