Word processors use various data structures to optimize operations like insertions, deletions, formatting, and undo/redo. Common ones include:

Gap Buffer: Efficient for insertions/deletions near the cursor.

Rope: Ideal for large documents, supports fast substring operations.

Piece Table: Tracks editing operations, efficient undo/redo.

Linked List: Good for line-based editing, fast insertions/deletions.

Array: Simple for small documents, fast read operations.

B-Trees: Efficient for large files, supports search and modifications.

In practice modern editors often combine these structures for better efficiency. for example : using Piece Tables for text storage and Ropes for efficient editing.