Open the Google Photos app on your iPhone
Tap your profile picture in the top right
Tap Google Photos settings
Tap Backup
Turn off Backup
Open the iPhone Settings app
Tap Photos
Turn off iCloud Photos if you also want to stop Apple Photos syncing
Open the Google Photos app again
Tap your profile picture
Tap Use without an account or sign out of the Google account
Delete the Google Photos app if you no longer want it on your iPhone
Remove Google Photos access from your Google Account if needed
Go to your Google Account security settings
Find Third-party access or Connected apps
Remove Google Photos access from the iPhone if listed