Drupal removed its automatic entity-updates API in 8.7. See https://www.drupal.org/node/3034742. so you can no longer run drush entup in your CLI,  at least unless you install this module https://www.drupal.org/project/devel_entity_updates 

If you are getting something like

To get all method names you can run on any field added to any entity, input this into devel/php

ksm(get_class_methods('\Drupal\Core\Field\FieldItemList'));

To get route name that you maybe want to use in some custom code you can get it with this 2 ways

So if you want for some reason to get a count for each of the entites in your DB, run this code and get some output for it

Drupal commerce has rather good documentation for D8 commerce but I will try to summarise here how each order is made, what it needs to be completed and what goes on in lifespan of an order.
So lets start by order entity it self, check

If you are using currency resolver module in d8 you might need to change resolved currency quickly go to

namespace Drupal\commerce_currency_resolver;

and in getCurrency method, put 

If you are having any problems with checkout flow (designing custom checkout flow or new payment gateway) check in 

Drupal\commerce_checkout\Controller;  

and see formPage method, debug it and you can track what is step sent from URL, what is step set in order and do they match and adjust accordingly when needed.

In each controller you make, there are special objects you can use to get data you may need. They are 

If for some reason your commerce site gets stuck while in development with some bad data for user cart, to get rid of all the carts you can fire this code to delete all the carts for that user (probably admin or user 1)