Backend is mostly about storying, querying, shuffling and processing data. Ask dba for books on sql, he will recommend you something - do some excersise on opti

Backend is mostly about storying, querying, shuffling and processing data. Ask dba for books on sql, he will recommend you something - do some excersise on optimizing queries for particular database to understand how it works. Get familiar with nosql solutions, study what each of them does best and how are they used. Practise using them to solve real-life problems.