Are smart dimmers compatible with Alexa, Google Home, and Apple Home?

Most are. Lutron Caseta works with Alexa, Google Home, Apple Home, and Ring. The TP-Link Tapo S505D adds Matter support including Siri, while Kasa and Treatlife cover Alexa and Google Home. The newer Leviton Decora Smart and Matter-based Tapo models extend to Apple Home as well.