Ok I bought a rear derailleur m9050 snd the speed shifter 9059. A battery and battery tray all cables from derailleur to 3 port junction to battery and to the shifter. But I have nothing am I missing a part? Also side not pretty stupid you order a charger and doesn’t come with cable to plug it in noticed that with almost every part I bought
The A junction is like the brain of the system. You need one. B junction is just a splitter, often placed around the BB, where the battery, RD and in some cases, the FD split (from the A junction).
If you have an SM-JC41 (B junction / splitter) only, then it would appear you are missing the A junction.