Module development and code questions

How can I change text of "create taxonomy term" button?

This is node reference field for taxonomy, appears when I click Add new taxonomy term when add or edit a node.

My objective: create a page.html.twig override for Vcabularies.

Intention: So that term page displays under each vocabulary looks different.

The code I am tryng to use in my themename.theme is this:

use Drupal\taxonomy\Entity\Term;

I'd like to add in the name field from an entityform to a mailchimp list via the mailchimp automation module, but I'm not sure how to code this.

From the configuration form that is loaded when a new block is being placed, or existing block configuration is being edited, how can one determine, programmatically, if the active form is for a new block or configuring an existing block?

This appears to be possible as the form itself changes the access to the machine name property between a new block and existing block, but I haven't been able to figure out what property it is looking at or where this part of the form is being built from.

When I find a function in the Drupal API, how do I work out how to actually use it?

This may not be the right function for doing this, but for example, I want to get a list of all the libraries that have been loaded to a page and looking through the API documentation, I found public function AttachedAssets::getAlreadyLoadedLibraries, how would I work out how to actually call this function in my code? 

Hi 

I have select box with allowed values and it has related many related existing content. I tried function hook_post_update_ function.