The source is the official play-by-play of each game, the same record the box score is built from. It gives the down, the distance, the yard line, the formation family, who threw, who caught, where the ball was caught, who made the tackle and the result of every snap, plus a win-probability estimate after each play.
Each snap is redrawn on the TacticalSnap gridiron board with the ball on its real yard line and the first-down marker where it was. Named players are the ones the record names; everyone else is a position placeholder. Routes, coverages and blocking schemes are not in the record, so they are not drawn or claimed.
The narration is written from that record and the published match reports. When a tactical explanation comes from a coach's quote or a film study, the page says so.