Cart-One page checkout: looking for uptodate documentation (if there is one)...

[aftertaf]

Hi,
Amateur, getting there bit by bit with my new site.... and learned a fair amount of random things.
I've been able to override product pages, categories, add-to-basket button - looking now to the onepage checkout.
For template, using one i built with TemplateCreatorCK (thx Cédric for such a useful tool).

Anyway,
i'd like to tackle the cart page, but i'm also wondering if there are ways to override any/all part of it without hitting the core files.
As this would lead to modifying quite a few files, i wondered if there were any guides or ducs. I've read and reread your threads on this subject but they started in 2012, some last updated 2015. Especially when it comes to OnePage Checkout....

Tooltip warns about 3rd party templates needing to support, but not much other info that i feel i can trust (dating to VM2).

What i've gleaned so far (by dropping the filenames into the files as commentary, and i'll add more as/if i figure anything out...) is the basic file structure.:

\YOURSITE\templates\YOURTEMPLATE\html\com_virtuemart\cart\default.php
   Line 55:       echo $this->loadTemplate ('shopperform');   <!-- cart\default_shopperform.php-->
   Line 67:       echo $this->loadTemplate ('address');             <!-- cart\default_address.php-->
   Line 69:       echo $this->loadTemplate ('pricelist');             <!-- cart\default_pricelist.php-->
      Line 140:       echo $this->loadTemplate ('coupon');                                                  <!-- cart\default_coupon.php-->
      Line 238:       echo $this->loadTemplate('shipment');                                               <!-- cart\select_shipment.php-->
      Line 282:       echo $this->loadTemplate('payment');                                                <!-- cart\select_payment.php-->
Line 83:       echo $this->loadTemplate ('cartfields');         <!-- cart\default_cartfields.php-->

Its more useful with the graphical attachments, i'll try to upload them with indication of each from my OneNote on this.

But if there is some doc somewhere that ive failed to search for, i'd appreciate.