The runner/starting block/ground/Earth system can be represented using an interaction diagram.
We will not use interaction diagrams very much in the course. There are only mildly useful in problem solving but they have their proper places in the textbook as they do help to visualize what is going on.
All the forces have third law partners but since we have only shown the free-body diagram of the runner and the block, we do not see the third law partners of the ground and Entire Earth forces (which would be on the free-body diagram of the ground and Entire Earth).
But the force on R coming from B and the forces on B coming from R are third law partners and we know that their magnitude are the same (but they have opposite directions).