The final chapter is the longest, and it works as a summation, revisiting nearly every theme in the book with the vocabulary of the three strands now fully available.
It begins by separating two words Arjuna has been using interchangeably. Renunciation is the giving up of actions prompted by desire; relinquishment is the giving up of the fruit of all actions. The chapter endorses the second and says explicitly that acts of sacrifice, giving and austerity should not be given up at all, since they purify the wise, though even they should be performed with attachment and the expectation of results abandoned.
Then comes a passage that is easy to pass over and is the closest thing in the Gita to an analysis of agency. Five factors, it says, are required for the accomplishment of any action: the seat of action, which is the body; the agent; the instruments of various sorts; the various and distinct activities; and the fifth, divine providence. Whatever action a person undertakes with body, speech or mind, whether right or wrong, these five are its causes. Anyone who therefore regards themselves as the sole agent, on account of an imperfect understanding, does not see rightly.
That is the argument the whole book has been assembling, put in the plainest available terms. It does not deny that Arjuna will act, or that the action will have consequences, or that some actions are wrong. It denies that a single agent is the whole cause of anything, which is what makes relinquishing the fruit a description of reality rather than a spiritual exercise.
The chapter then sorts knowledge, action, agent, understanding, firmness and happiness each into three kinds, applying the scheme of chapter fourteen exhaustively.