Sounds easy if all you have to do is write a short comment about it. People who actually did it, like the recently retired lead of the Jellyfin project, didn't make it sound like it's "not that difficult" [1].
If you want ot vibe code a project for yourself it's probably reasonable amount of effort. But if you want to build a product, something with polish, something not held together by spit and scotch tape, something reliable, it won't be easy.
From personal experience, the transcoding was the easy part. Building a UX that was intuitive and a backend that catalogued the media accurately required far more attention.
Making something that actually interacts with "the human world" is 100s of times harder than writing any kind of algorithm.
I'd challenge that. Have you looked at the specifications of any reasonably current compression standards? The AVC one is some 900 pages, I think.
Entire communities exist around discussing the strengths and shortcomings of different codec implementations.