Multiple [Quality] Versions of the Same Movie

[ThomasPGH]

Is Jellyfin able to select the best version it doesn't have to transcode? Or does it take your selection and automatically transcode if needed? If the latter, then might as well only keep the highest quality UHD->BD->DVD.

No. Before 10.9 Jellyfin chose at least the 1080p version of a film over 4K on mobile devices as the default unless a different version was picked manually. Since 10.9 all clients default to 4K, resulting in downloading tens of gigabytes over 5G if you don't pay attention.