If multiple Google Accounts have been added or linked on the OnePlus smartphone, we can always choose the backup account where the data backups will be stored.

Android 10 (OxygenOS 10.0)
Step 1:  Open the  Settings
Step 2:  Tap on  System
Step 3:  Tap on  Backup
Step 4:  Tap on  Account
Step 5:  Choose a  backup account
  1. Open the Settings
  2. Tap on System
  3. Tap on Backup
  4. Tap on Account
  5. Choose a backup account

OnePlus Instructions