How are weight updates handled in Batch Gradient Descent vs SGD?
What is being stored at each sample, that can be applied at the end of the batch

You store the prediction and error and then calculate the average after each cycle.

GD - Update after all rows. It will give a correct direction to the weight update but will be very slow

SGD - Update after each row. It will be fast but the direction may swing

Batch - Update after a specific count. This approach gives a balanced solution to the two other approaches.