Sign out of all Google accounts in your browser
Open the Google service you want to use
Sign in with the Google account you want as default first
Add any other Google accounts after signing in to the default account
If needed, click your profile icon and verify the first signed-in account is the one you want as default
In Chrome, go to Settings
Under You and Google, sign in with the preferred Google account first
Remove other Google accounts from the browser if they keep becoming default
On mobile, set the preferred account first in the Google app or browser
Reopen Google services and confirm the correct account is selected by default