They might have several possible formats but for example netflix branded content specs are in the net [1]. It’s IMF package with jpeg2000 content. Prores files are sent as helper files. Sometimes ffmpeg can be used but often grading / mastering tool is used for export (resolve, baselight etc). In my experience FFMPEG is very easy to keep consistent once you get the parameters right (which is always pretty painful), but pretty slow compared to most other tools.

[1] https://partnerhelp.netflixstudios.com/hc/en-us/articles/726...