Avoiding transcoding ...

[jellyfin]

First let me say that the above link says that h.265 / HEVC on Safari or iOS is "is only supported in MP4, M4V, and MOV containers."
That does not seem to be true!
-> I can play H265 from a MKV container and it will be shown as "direct" in the JF dashboard! 

I also want to point out that Jellyfin will show "direct" next to the video if it's not being transcoded.
You're misinterpreting things though. It's just saying that the video stream is direct, not that the whole container is being direct played.

No, I did not misinterprete.
I know this and I noticed the "remux" in the playback information - also the files in the transcoding cache.
What I tried to say here is, that from the Codec information page, H265 should NOT directly playable from a MKV container - but it is (with remux because of the container).
At least, this is how I read the table.
More clear?

But then, h.264 should work in any container.
That does not seem to be true!
-> I can play H264 from a MKV as "direct" but it still get's remuxed!

Something is wrong here.

For this, I created both H264 and H265 videos in a MKV container - and the Dashboard in both cases says that the video is played "Direct".

Also, in both cases, the video get's remuxed - because of the container!
I repeated the same with videos without any audio and then even without any subtitles, just to be sure - same behaviour!

Nothing is wrong. As you stated before, Safari on iOS only supports some codecs in MP4, M4V, or MOV containers.
Jellyfin is taking that into account to make sure you can direct stream your video.

It just switches the MKV container to one that is supported.

If you want to avoid this specifically on Safari on iOS, switch your containers to MP4.

A lot of things in life has trades and balances. By switching to MP4, you will be losing out on some other things such as not being able to include certain audio or subtitle tracks along with the video as what MKV provides.
Just let Jellyfin transcode/remux when it's needed for you. That's why Jellyfin has that capability. It's to make your life easier instead of having to worry about making all your files compatible with every single device.

No, I sure don't want Jellyfin to either transcode or remux anything!
Instead, I will need to re-encode Terabytes of data, which let's me freeze in horror.
I need encodings that will not need transcoding or remuxing anymore ....

And just in case, and to repeat again, the most compatible format for most clients is as follows:

Video: H264 (AVC) 8-bit
Audio: AAC 2.0 (128kbps total / 64kbps per channel)
Subtitles: SRT
Container: MP4

Thanks for this, this ensures me in what to do best with my stuff.

So, for DTS 5.1ch, AAC with 384 would be best?
This is only half or a quarter of the DTS tracks, which seems ... a bit low.
EDIT 2: That does not seem to be needed, as DTS can also be played directly when in a MP4 container! No transcoding, no remux. I stay at passthrough as much as possible and will only otherwise use AAC with 64 per channel!
EDIT 3: I tested this and as it seems, the Jellyfin client on iPadOS cannot play DTS from MP4 without transcoding!

I need to recover from this new information about MKV always needing remux.
That was a hard strike.

EDIT: For some reason either my Mac, Safari or this webpage enabled "automatic corrections" so that all my typing of "remux" got changed to "remix".
I tried to fix that.