Most databases use B+ trees, not B-trees. The key difference is where data lives.
B-tree: Data stored in both internal and leaf nodes.
B+ tree: Data only in leaf nodes. Internal nodes only store keys. Leaf nodes are linked.
Why B+ trees win:
More keys fit in internal nodes (better fanout)
Range scans follow leaf links without backtracking
All leaf nodes at same depth (predictable performance)
Sequential scans are much faster with linked leaf nodes.