FFmpeg
FFmpeg is a collection of libraries for handling audio and video. It is used primarily in Linux, however ports for most other operating systems exist. It's often considered the "swiss army knife" of simple AV manipulation, being able to convert between tons of file formats, along with manipulating subtitles and simple effects (like transformations).
https://ffmpeg-batch.sourceforge.io/ - A GUI frontend for FFmpeg with a focus on batch conversion
Usage
FFmpeg is, standalone, a CLI (command line) only tool.