I believe subtitling is done from the script for a show, which is why sometimes what is on the screen doesn't match what the actress says. Typically, it's the same concept, just with slightly different words. They probably just load it up and make sure it's synced.

CC (I think) is done live by transcribing what is actually said. That's certainly true for live events where there is no script.

There were also issues with diagrams, charts, maps, tables, etc presented in the videos that should have links to screen-reader friendly versions for the blind, or audio descriptions.

There may be legal matters of certification. Since CC is required by law, it probably has to meet certain requirements. You probably can't just hand the job off to some native-English-speaking teenager armed with Subtitle Workshop. It likely has to be someone who knows what they are doing and whose work is somehow validated for some sort of compliance. I would imagine.

There seems to be a lot of duplication too, some courses have 4 different copies uploaded from 4 years.