Learning about Transcoding/ Bitrate

[MChrisp]

Thank you for the answer, apologies further stupid questions:

HEVC 8bit and 10 bit -  I assume one just offers a higher quality?

With Jellyfin determining if a transcode is needed, why does some of my H265 get transcoded and others don't, I would of assumed they were the same?

Is it worth converting 264's to 265's with Handbrake or am I just saving on disk space?

8bit and 10bit (there is also 12bit) refers to the color depth, so basically how many different colors you have. Standard is 8bit. If you have 4k HDR content, it is mostly if not always 10 bit or even 12 bit if you have dolby vision. (In case you don't know what hdr is: Basically it tells you player what the brightest and dimmest spot of the movie is. in some variants, like dolby vision, this is per frame and also gives you better color)
If you transcode, just use x265 encoder. This is broadly supported and you will probably have no problems. If you have older hardware (meaning your player/TV or/and you encoding PC) use x264 encoder.

There are presets for the encoding speed. In general just use as slow as you can, that improves quality and/or lowers the bitrate/filesize. You have to figure out, what works for you.
For the quality settings of the encoders: Use the presets. They should be around 20 for x264/x265. Lower means better quality, but larger file sizes.

For Audio: I came to the conclusion, that dolby digital is your best bet. It has decent sound quality up do 5.1 channels. It is basically supported by everything. AAC is mostly fine. For me it does not work for my TV/AV for some reason. For this select highest quality basically. It is practially irrelevant for file sizes. And you would not want bad audio quality.

Then there is also the container type. e.g. .mp4 or .mkv
This is what packages the audio, the video and subtitles, title picture etc. into one file. mp4 is in general better supported by players. But if mkv works for you, i would suggest using mkv. Also remuxing (changing the container) can be done easily by jellyfin on the fly.

In generall I would not suggest transcoding h264 to h265 if you already transcoded h264 to safe space beforehand. If you are talking about transcoding a bluray file (which off cause would be a not copyrighted movie ) use x265 if you can.

If you have very new hardware (which you probably have not, given your PC-stats) you could use AV1 encoder. It is extremely efficient and gives very small file sizes with very good quality, but is not supported by many players and also takes a long time to transcode. (If you have an AMD ryzen 9700 or another very potent CPU, it is worth it, if you player supports it.)

What exactly is your use case?