What exactly happens if an unauthorised user attempts to access my accounts?
Some applications will notify you by text, email, or an authentication app, stating that someone is trying to log into your device. If this is you, or someone you have granted permission to, you will be prompted to enter your secondary authentication on that device.
It should always be down to you to physically enter the secondary step, and you should never hand over this information to anyone else.
However, if you get an alert, and you haven't permitted anyone to use these accounts, you will state that it isn't you trying to log in. The unauthorised user will then be rejected from gaining access.
As the hacker will have managed to get hold of your password, you should then change your password. Remember to create a strong password to reduce this risk of this happening again.