While learning about segment tree I came across this problem:-

The task is as follows: for a given value x we have to quickly find smallest index i such that the sum of the first i elements of the array a[] is greater or equal to x (assuming that the array a[] only contains non-negative values).

Link-https://cp-algorithms.com/data_structures/segment_tree.html

The solution mentions storing prefix sums in the segment trees.

I cannot figure out how to build up a segment tree of prefix sums. Also how is it different from the range sums?