Connecting To Loan Company API

Yes, easy to do.
You put your code into a plugin and access it using shortcodes.
This plugin has examples of using forms to capture data that gets submitted to the server, in your case this would be the API.
https://wordpress.org/plugins/wp-csv-to-database/
Using shortcodes here:
https://codex.wordpress.org/Shortcode_API

Not sure that there is that much to know.
I am sure there is a doc somewhere, not sure I have read it. Found it now, on this page: https://codex.wordpress.org/Main_Page
Is a section “Write a Plugin” with three parts:
https://codex.wordpress.org/Writing_a_Plugin
https://codex.wordpress.org/Plugin_API
https://codex.wordpress.org/Plugin_Resources

When I started I had much more experience reading PHP than writing it. Please read the sample that launched me:
https://wordpress.org/plugins/wp-csv-to-database/
Yes it has nearly 700 lines, but spend 5 minutes scrolling through it, plant memories like “so that is how xxx happens”, so that you can get back to it later.
You will see that there is a php file with the same name as the directory, so give your plugin a distinct name.
It is possible that you don’t need any options or configuration for the plugin, so all that can get removed.

//your shorcode stuff need not have anything to do with much else, may I suggest
// something like this:

class ABC_class {
	private $varA;	//all the classes private variables
	private $varB;
	public function __construct() {
		$this->varA = "";
		$this->reset();
	}
	private function reset() {
		$this->varB = "";
	}
	private function support_fn() {
	}
	public function do_foobar() {
		//the shortcode gets done here
	}
}

//[foobar]
function foobar_func( $atts ){
	//supply custom loan API
	$mru = new ABC_class();
	$mru->do_foobar();
}
add_shortcode( 'foobar', 'foobar_func' );