node_delete_confirm_submit() and hook_node_delete()
I've found inspecting the routes availble and then working back to the controller to be a super helpful way to understand how to override things.

You will need Drupal console installed.

For example run 

drupal router:debug | grep node | grep delete


This will list all routes, and then filter only ones with node in and then filter only ones with delete in. 

You should then see the route you are interested in (/node/{node}/delete) is called entity.node.delete_form

Now you can debug just that specific route:

drupal router:debug entity.node.delete_form


This will show you that the form for this route is called node.delete. If however there was a method and a controller for the route is would show you exactly what the callback is.

To change the entity.node.delete_form route to use your own callback you can use the new method of doing menu_alter.

However if you want to keep the same node delete form and just change the submit handler then you can still use hook_form_alter to change this.