want to make one's own converter

For example, for video and audio files, there are (free) conversion tools such as FFmpeg and ImageMagic for image files.
However, they are basically operated on the command line, so it is difficult for unfamiliar users to handle them, so there are many GUI apps that call FFmpeg and others internally.(HandBrake, Xmedia Record, etc.)

Some files that can be converted online are probably behind the scenes using these conversion tools.

It's hard to create a "convert files to different formats" mechanism from scratch, but if you're "receive files and convert them with existing tools," it might be a little easier to create.

reference:
Manipulate various videos with the FFmpeg API -Previous