To switch back to "OK Google" (Google Assistant) instead of Gemini, open your Google app settings, navigate to "Google Assistant," then under "Digital Assistants from Google," select "Google Assistant" to choose it as your primary digital assistant; this will allow you to use "OK Google" to activate the traditional Google Assistant instead of Gemini.
Key steps:
Open the Google app, Tap your profile picture in the top right corner, Go to "Settings", Select "Google Assistant", and Under "Digital Assistants from Google," choose "Google Assistant".