How pattern matching works
Every crossword answer has a fixed length and, once you have a few crossing letters, a fixed set of known positions. The solver treats your input as a template: each real letter must match exactly, while each ? can be any letter. It then scans the dictionary for words of the same length whose known positions line up perfectly with yours.
This is fundamentally different from an unscrambler or anagram solver, which work from a bag of letters. The crossword solver cares about position: where a letter sits in the word matters just as much as which letter it is. That is exactly what a crossword grid gives you — letters locked to specific squares.