how can I go from backend dev to distributed systems engineer how can I fill the gap between them ?

Distributed Systems needs to be studied, there are some theoretical concepts & knowledge that needs to be acquired correctly, so start by finding some reputable books and courses, and study them, build things, and apply the knowledge, it will take some time Ahmed Farghal has a good playlist to get a feel in a friendly way about some of these concepts, but I also encourage you to find some courses like this one
https://www.youtube.com/watch?v=cQP8WApzIQQ&list=PLrw6a1wE39_tb2fErI4-WkMbsvGQk9_UB&pp=0gcJCf8COCosWNinmAkA