I don't understand where you are coming from.
HDMI is a serial digital point-to-point interface (ie one machine's output goes to another machine's input and nowhere else), and sends 24-bit colour information for every addressable point in the image - as such the picture received and displayed is exactly what was sent.
RCA is the name of a coaxial connector rather than a comms system, also known as "phono". Sometimes there are five connectors in the set for RGB video + LR audio, but if there are just three that's composite video (yellow connector) + LR audio (red and white connectors). RGB is better quality than composite, and can accommodate various scan resolutions much the same as a computer VGA interface to the monitor. However, these are still analogue signals (with sync signals carried on the green channel).
Composite uses an encoding scheme to add colour information to a luminance signal (effectively the black & white picture). If you were to view the composite video without extracting the colour signal, it would be monochrome. The method used to "hide" the colour signal inside the luminance signal is what distinguishes PAL from NTSC, and these rely on a high frequency sub-carrier that has little or no effect on a monochrome display. As such, the bandwidth of the luminance signal is restricted to substantially less than the chrominance sub-carrier, and a composite video connection only carries video at a resolution equivalent to "old-fashioned" 625-line analogue broadcast TV.
The Humax HDR-FOX and HD-FOX have composite video RCA connectors (plus left and right audio).
The SCART socket is another matter (AKA Peritel). This is again a point-to-point interface, but has 21 pins to play with. Connections are available for input and output (simultaneously) of LR audio, composite video, and RGB/YCbCr video. YCbCr is an alternative method of encoding the colour of each pixel more akin to the composite system (Y is the luminance and Cb, Cr are the two colour difference signals). Each is as good as the other, and often selectable by option on the equipment sending the video.
Display equipment (the TV) will just take the composite signal if that is all it is able to do, but if possible it is better to switch it to the RGB/YCbCr signal set. Because these send video over three separate connections without sub-carriers, they are capable of much higher quality and higher resolutions.
Thus, whether SCART is better than RCA depends on what facilities are available and how you use them. For modern TV and video equipment, the best choice is HDMI every time. If I had to, my next choice would be component video on RCA connections (five of them, not three!), and then RGB over SCART. Only after that would I consider composite RCA (three connections) or composite SCART.
My comments regarding cables with an HDMI plug on one end and analogue connectors on the other is simply that just making and selling such a cable to the general public is tantamount to fraud - these cables have no purpose, even though Joe Bloggs might think they do because they exist. Connecting an HDMI input or output to analogue outputs or inputs won't work, and in the worst case could cause damage.