[noggin]

Ok, very good explanations.

One last understanding question:

I have at home the one or the other BluRay. I was playing with HandBrake and other apps. But do they really deliver 4K/HDR Output or is it just a "copy of the screen"? In other words - makes it sense to backup my BluRays and getting better quality for audio/video compared to the verisons on Netflix/Amazon?

Br,
Johannes

If you rip your Blu-rays losslessly - using MakeMKV or AnyDVDHD - then you should get identical quality to playing the disc in a player (as the rip is a lossless copy).  If you decide to reduce the amount of data required to store a disk by re-encoding (using HandBrake) then you will be potentially reducing the picture quality, but whether the loss in quality is visible is a separate matter.  Lots of people find a re-encode to a lower bitrate still delivers the quality they want (and still higher quality than many OTT services) and prefer to have to store smaller files.   

Thanks, last question: What happens with HDR in this case (lossy vs. lossless)?  

If your are ripping UHD Blu-rays then it will depend how you rip them. HDR10 should survive all lossless ripping solutions. Dolby Vision is more complex - and no current Kodi solution that will play the UHD BD flavour of DV (which is a dual-stream variant), however DV UHD BDs have an HDR10 basic stream, and a lossless rip of that should be fine as an HDR10 rip.

If you transcode to reduce your bitrate you will need a transcode solution that is HDR-aware and preserves metadata etc.