Truenas apps are just docker containers that were written by someone else anyways. You can always just turn them into a custom app and see it’s internal composition, o just make a custom app and choose the image and settings yourself exactly like in portainer.

Might be a jellyfin setting. I discovered that while the players report the device capabilities (with some caveats), some edge cases make the server still transcode video because it instructs certain formats to always transcode. There are too many variables and the server plays it safe. The biggest culprit is bitrate.

Yeah, that is the kind of concern for the service developer or a very opinionated sys admin. For self-hosting, few people will reach the workload where such a decision has any material or measurable impact.