FCFF/FCFE, intuitively net, investment, different, difficult, available

PieMan

When solving for FCFF/FCFE why are we adding back 'Int (1 - tax rate)' and 'net borrowing' respectively? Tax shield, money available to equity holders, etc. Understand the fixed capital, working capital investment aspect, but the debt part is not sinking in.

It's going to be difficult to memorize all the different derivations so need to get better understanding of what's really happening.

Thanks,

John