For example, if my main account uses a very secure password, and I never sign in using this user name and password inside of a game or mobile phone app.
However, if I have a less secure account on Google, and use it to sign into games and apps on the mobile phone, and then I use "Add an account" in Google, then, anybody who can log into this less secure account can also get a hold of my main account and see all my emails. For example, if it is a game on an Android phone, and it asks me to log in and I use this less secure account, and somehow that app has a trojan horse and get my password. Then it can now access my main, important account too.
Is it true, and if so, then it seems we should not add any less secure account to our main account, and perhaps use an Incognito window and only "Add an account" and use these less secure accounts together. (and that these less secure accounts never have any important personal info or email).