Some questions to the Snipcart Plugin

I would love to help, if I can.
What is a PR? Personal request? If yes, where?

I’ve come across some other basic questions:

Where is this.$api defined? Maybe you use Axios somewhere but I can’t find it.

I guess the path 'snipcart/orders' of the get request is defined here:

But what means (:all) (1)?
And where is $params (2) defined?

I would like to do a put request to update an order.
How can I do that?