How to select a whole.

I have a face i placed into a helmet and i need to get the head back out.
Is there a keybind i can use that when i select like a vertex then press the keybind it will automatically highlight all the remaining vertices on the object whole?

Thanks in advance for any insight on this issue.

That will be [CTRL L]. (Menu Select --> Linked) But you don’t even need to select, hover a vertex, an edge or a face (depending on your selection mode) and press [L] to select all linked parts. ([SHIFT L] to deselect.)