Wikipedia, "Pushdown Automaton" — a finite automaton with an auxiliary stack that recognizes context-free languages.
https://en.wikipedia.org/wiki/Pushdown_automaton

Tutorialspoint, "Non-deterministic Pushdown Automata (NPDA)" — explains NPDA functionality and how multiple paths are explored during recognition.
https://www.tutorialspoint.com/automata_theory/non_deterministic_pushdown_automata.htm

OpenDSA (Virginia Tech), "Equivalence of NPDA and CFG" — course notes demonstrating that any CFG can be converted into an NPDA and vice versa.
https://opendsa.cs.vt.edu/ODSA/Books/CS4114S23notes/html/PDACFL.html