West's 3♠  is 'bidding to the level of the fit'. In competitive auctions you're usually safe to bid to make a number of tricks equal to the number of trumps that you and partner hold between you. East wouldn't overcall with fewer than five Spades so West knows they will have nine trumps between them if they play in Spades

Most declarers in a Spade part-score got a Heart lead, normally the ♥ 7 (highest card when leading from a 2-card suit) but then failed to make nine tricks.

The trick is in spotting the danger of a Heart Ruff later in the play and so ducking the first trick. If you win the ♥ A then when you lose an inevitable trump trick South can win ♠ K, give North the lead with the ♥ K, take a Heart ruff before cashing the ♠ A and ♣ A to tally five tricks for the defence.

If you duck the first Heart trick then you keep control and the only Way North can get the lead to give South a Heart ruff is to win the first trump trick with the ♠ A. You don't care about South ruffing with the ♠ K, which is the only trump they have left, because that card was always going to win a trick.

In 6♥ by North East should lead their singleton Diamond for West to win and give them a Diamond ruff for the setting trick = one down

6NT by South doesn't look promising on a Club lead but if West wins and immediately plays off Six Heart winners East must come down to six cards and is under pressure.

East cannot keep five Spades, a winning Club and a Diamond.

If East throws a fifth Spade that gives Declarer five Spade tricks to add to the seven they have just taken = contract made

If East throws a Club that gives Declarer the ♣ 10 and four top Spades tricks plus the seven they have just taken = contract made

If East throws a Diamond then Declarer plays a low Club which East must win before giving the lead back to Declarer with a Spade to cash Four Spades and the ♣ 10 to make their contract