We were clueless the first time we hooked up our N64 at gran-gran, since the old TV did not have a Scart connector, but we figured out that the Scart’s colored cables go in there.
Theoretically it had all that in one cable, in practice it never did. You’d usually have 3-4 SCART ports on a TV, but not all ports accepted our output the same signals. There was no way to tell from the outside what the output or input from a SCART port so you either had to try different port combinations or look it up in the manual (if you had one). Most TV’s had one port that accepted s-video, on that accepted RGB and they usually accepted composite on all ports.
Worse, not all cables had all 21 connections. If you were lucky you could tell because not all pins on the connector would be there (but this wasn’t necessary the case).
Usually there was also one port on a TV that output the video from the tuner. This was used for analog pay TV decoders. You would hook it up to that SCART port and it would get the scrambled video from the TV and return the descrambled video over the same port.
Also, due to the size and design of the connector it was almost impossible to insert it blindly. Inserting one into the back of one of those enormous CRT television was always a challenge.