Smart Bidding is Google setting your bids automatically, auction by auction, to hit a goal, maximize conversions, target cost-per-action, using signals no manual bidder can see. Use it once it has food: it learns from conversion data, so it needs working tracking and ideally ~30+ conversions of history.
Before that history exists, automated bidding is guessing with your money. A new account often does better starting on capped manual or Maximize Clicks while tracking accrues, then graduating.
One behavior to expect: every major change (new conversion action, big budget shift) triggers a learning phase. Up to a week of wobble. Change things in batches, not daily tinkering, or the algorithm never leaves school.