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