Thanks for reaching out and for your interest in syncing your smart bulb and switch to the Google Home app.
A few questions: What's the manufacturer of the smart bulb and switch? What's the model of these devices? Have you tried linking them to the Google Home app before?
Made for Google and Works with Google light bulbs and switches are compatible with the Google Nest and Home devices. These devices can be synced and managed in the Google Home app. You check if your smart bulb and switch can be linked in the app. Explore this list of supported smart home devices.
Let me know if you have additional questions in mind.
Got it! If you directly setup your smart bulb and switch in the Google Home app, you might need to remove and set them up first in your external app before linking them to Google in order to manage them using both apps. However, if you used the external app to configure the devices before you connected them to Google Home, then there's no need for you to do anything. As long as your external app is still installed on your phone, you can manage your devices there at any time.
Fill me in if you have further questions.