What happens if a developer in my team is truly underperforming?

First, you should inform your Project Manager and share your doubts and findings. Then, the PM is organizing an 1x1 meeting with the developer to have a better understanding of the situation and discover the reason for underperformance. If the situation is critical, also a dedicated person from HR can be engaged by the PM. In most cases a conversation is the first step that helps; then we can all agree on a ‘repair plan’ with performance goals and deadlines. If it won’t help we’re engaging your Delivery Manager and will start looking for other solutions, e.g. new developer to join your team.