[4fterd4rk]

Great explanation, but the last question is quite simple. You determine the weights via brute force. Simply running a large amount of data where you have the input as well as the correct output (handwriting to text in this case).

[ggambetta]

"Brute force" would be trying random weights and keeping the best performing model. Backpropagation is compute-intensive but I wouldn't call it "brute force".

[Ygg2]

"Brute force" here is about the amount of data you're ingesting. It's no Alpha Zero, that will learn from scratch.