If this is a serious question... you'll pay a (small) fraction of a bitcoin, just like you'd pay a fraction (or some units and a fraction) of whatever currency.
Just as when you pay with a card or online, you just pay the amount. There's no change.
Just as when you pay with a card or online, you just pay the amount. There's no change.
And then you get millisatoshis that can be used on the lightning network, but I've not dealt with that directly myself.