Unattended access question

Dear experts,

I have just started using teamviewer. During a remote screen session with a local vendor who was helping me troubleshoot my OS I noticed he setup unattended access on my machine. I tried researching a lot about this and am only getting more worried. Please help me with my questions.

a. Do I need to be logged in to the team viewer client for him to get access to the machine? Or is it possible to remote access it even when I am not online in Teamviewer.
b. Is there any chance he can login remotely to my system when it is sleeping/hibernating ?
c. Is there any chance he can access my machine without my knowledge of his activities ? If so, does he need my OS user credentials for it or can he access it without those.
d. With the unattended feature setup will he be able to use my machine as a terminal, as in use simultaneously without me basically having an knowledge of his activities?

Appreciate all the time and support.

Thanks and Regards,