Currency symbol is not stored in order, so to get it, you need to dig a bit deeper and fetch currency object.
I am assuming you have order stored in $this->order so then we get currency code and with that we load commerce_currency through entity type manager and use getSymbol method to get the symbol.

You maybe added some patch to composer.json, but it didn reflect in composer.lock when you ran "composer install" so to force composer lock to update you should run

composer update --lock

If you wan to get coupon entity from order there is a quick magic method to do that. First load order by some property, we can use ID
 

You want to get exact class for particular plugin field.type, type in this in your devel for "entity_reference"

$field_type_manager = \Drupal::service('plugin.manager.field.field_type');
$class = $field_type_manager->getDefinition('entity_reference')['class'];

this will show you info

In drupal 7 when you moved modules from one directory to other, usually running drush rr (registry rebuild) would work, maybe you would need to clear caches. In drupal 8, this wont help. Paths are saved in APC and you will probably need to restart server.
For someone maybe 

For selenium there is also selenium command runner, called selenium side runner, as I pointed out some tips about how to use NEW Selenium IDE here, here are tips on how to use it with CLI or with some CI/CD.

As it is often referred online as "new selenium IDE 2019" lets sum up some tips how to use it. It can record your actions and put them in its UI, but you will often need to change it and refine it to get something useful.

If you used google pagespeed and got "Enable Text Compression" you will fix this with uncommenting this lines in your nginx.config (or adding them)

Not many tutorials cover this often used practices in drupal 8, so check the differences betweeen self and this
https://stackoverflow.com/questions/5197300/new-self-vs-new-static

I was wondering how PHP gets compiled to OpCodes and how does that all work, there is a nice post that talks about OpCode caching in general (a bit outdated now) and also gives a good example of how php code is tokenized for OpCode https://support.cloud.en