Running balance

Task 1
Two columns A and B. A1 data entered, B1 calculation in cell. Is there a way when i enter data in A2 that B2 will populate using the formula from B1 and so on for the remaining cells in each column.

Task 2
Once column C hits zero and column B eventually total to $1,000,000 if there is a remaining after $1,000,000 the balance will go to column H and subtract from H3 and continue remaining balance like column C.remaining balance chart.xlsx

Sorry all but this my first time posting. A sample of the spreadsheet is attached with notes. Any help is greatly appreciated. Thanks