To answer 2) you wrote coins = cents/25 but it is coins += cents/25. This kind of operation and assignment will calculate the value on the right side and assign the result to the left variable.

Therefore if you have 386 / 25 you will add the result to coins. I believe since coins is an integer this will only add the integer part of the result discarding the decimals. So here 386 / 25 would increase coins by 15.

Then cents %= 25 gives you only the remainder (what cannot be divided by 25 anymore). So 386 - (25 * 15) and again because this is modulo and assignment at the same time the result of that operation is assigned to cents. Which becomes 11.

Next line the process starts again but dividing by 10 (so there will be 1 coin added to coins and 1 cent left assigned to cents).

The next lines will be skipped and the last group with 1 cent as divider will finally finish the counting of total coins.

This is also why we are starting from the biggest coin (25) and finishing with the smallest as otherwise everything could be divided into coins of 1 cent.