Thanks @Jitendra_jangid, I have already declared a global variable using the following syntax
Set(varManagerEMail, Lower(Office365Users.Manager(User().Email).Mail))
And this variable is now properly populating in the Combo box and the text on the button "Your supervisor is...'.
But when I added Set(varManagerEMail, DataCardValue1.Selected.Email) in OnChange of my combobox, the varManagerEMail gets resets to blank, and the default item that gets populated in the combo box is the first email address in my AD list, and not the value of varManagerEMail that I set OnStart of the App.
Other thing is, if the user does not change the default item in the combo box and saves the record, he sharepoint list does not update the supervisor email/name. I have the following code in UPDATE of the DataCard of the combo box. And that I found is because DataCardValue1.Selected.mail stays blank in this case. If the user changes the value of the combo box, then sharepoint list column value updates properly.
If(EditForm1.Mode = FormMode.New,
{
Claims:"i:0#.f|membership|" & DataCardValue1.Selected.mail,
Department:"",
DisplayName: DataCardValue1.Selected.DisplayName,
Email: "",
JobTitle:"",
Picture:""
}, DataCardValue1.Selected )
Hope this makes sense?