I want to customize the community login page. Can someone tell me what page/component/aura component that is this portion of the login?
You would have to enable the loginForm component, which is
Read the first line of the component, which says
add implements="forceCommunity:availableForAllPageTypes" to surface the component in community builder
So your second line looks like this
<aura:component controller="LightningLoginFormController" implements="forceCommunity:availableForAllPageTypes">
From here you can make any customization you need and use this Custom Login Form in your community, just as a custom Lightning Component.