Scan the indicated amiibo while playing the game (not through Uncle amiibo) by holding D-pad Right and placing the amiibo on the Switch to unlock the corresponding bonus. This can be done at any time, usually with no time limits or restrictions.

Highlight purple coins on-screen (not on your map): Any Bowser amiibo.
Life-up heart: Any Peach amiibo.
Random coins or bonus hearts: Any amiibo or amiibo card.
Temporary invincibility (30 seconds): Any Mario amiibo.