Before moving on, implement a BIT from scratch without looking at notes:
Implement update(i, delta) and query(i)
Build a BIT from an array
Test with range sum queries
Verify against brute force
Once you can write it in under 2 minutes, you have internalized the pattern.
// Template to fill in:
class BIT:
function init(n):
// Your code
function update(i, delta):
// Your code
function query(i):
// Your code
function rangeQuery(l, r):
// Your code
The best way to learn data structures is to implement them from memory.