Where is the B-tree data structure usually implemented and stored?

The data structure B-tree is usually implemented and stored in database management systems (DBMS) and file systems, where efficient search, insertion, and deletion of data on disk are required.

In Python, a B-tree can be implemented as a separate class or module that stores tree nodes in RAM or on disk. B-trees are often used for indexing large volumes of data to minimize disk read operations.

An example of usage is the btrees library in ZODB (an object database for Python), where the B-tree is stored as objects optimized for fast access and modification.

Thus, B-trees are implemented in places where high performance is important when working with large data volumes, typically in data storage layers.