Sign out of all Google accounts
Sign in to the Google account you want as default first
Open Gmail or another Google service
Add any other Google accounts after the first one
Check that the first signed-in account is the default account
On mobile, remove and re-add accounts if needed so the desired account is added first
In Chrome, set the preferred Google account as the profile’s main signed-in account
If needed, use a separate Chrome profile for the default Google account