Difference between $query and $the_query

There is no difference. Both are variables and the names you give those variables are a matter of personal style.

That being said, in the WordPress dev community, those are the typical names given to the variables used to hold a new WP_Query object.

Personally, I usually name the variable according to the query I'm performing. For example, if I'm querying a custom post type called 'services', then I will name my variable $services_query.

Here's a new WP_Query object being created to hold all posts in the services custom post type:

$services_query = new WP_Query( 
  array(
    'post_type' => 'services'
  )
);

Many choose to hold their arguments in an $args variable first:

$args = array(
  'post_type' => 'services'
);

$services_query = new WP_Query( $args );

Those two code blocks are identical. Personally, I prefer the first. I first it more readable.

Once you have your new object, you can use it to create a custom loop to actually print out the data associated with each post you've grabbed with your new WP_Query object.

All of this is found in the codex here - https://codex.wordpress.org/Class_Reference/WP_Query

You'll see in their examples, a new WP_Query object is held in variables like $the_query, $query1, etc. It doesn't matter what you name your variable, to the extent that it's readable and clearly conveys what is being held in that variable.

I should note that WP_Query is class used to create objects. This is Object Oriented Programming in php and an understanding of OOP will help you understand WP_Query a lot better.

I hope that helps.

In my experience it's hard to find the ideal starting point. I found myself going back and forth between other php frameworks, WordPress and more. Each time I get a deeper understanding. You may find the same thing.

You don't have to understand WP_Query deeply, or other aspects WordPress for that matter, before you can use it effectively. Don't wait to get a so-called 'expert understanding' before diving in and using this stuff; just make sure your understanding deepens over time.

So I disagree that you should wait ;) In my humble opinion, the path to expertise isn't linear.