Blackjack Blackjack strategy questions- Please answer

Hey I was wondering if anybody could provide me with the answers to the following....:

In four deck BJ, after the deck is swapped for a new deck, does the counting restart from 0?
(It does right?)

If the dealer shuffles the decks and replays them, will counting cards still apply?
(When won’t it apply?)

If one plays for hours together, the profit margin increases through steady profit over time at the table? Even though the cards are being shuffled?

More than once my count was 15+ but poor low cards were shown. So I guess it’s not always that one wins counting cards.

Source.