Adding id to event html tag or adding extra sub text to an event when displayed in timeGridDay

[ildebr]

Is there a way to add an id to the article tag that contains an event or alternatively is it possible to alter the structure of how an event is displayed in a timeGridDay view to include some extra text present in a subfield in the extendedProps fields present in an event?

[vkurko]

Is there a way to add an id to the article tag

Unfortunately no.

is it possible to alter the structure of how an event is displayed

Yes, please see eventContent option.