Laravel function to add a supplier, and associated addresses and contacts
You could use events for triggering these kinds of things, but there is a danger that your code starts to become harder to follow. Martin Fowler gives a good top-level talk on it here.

There are two issues jumping out at me:

Firstly, your method accepts a request object so there is no indication as to what the requirements of that method are. You could change the method to accept specific arguments such as $created_by, $address_details, etc. This would make it easier to re-use the code elsewhere if needed.

I would make the controller then responsible for preparing that set of data (e.g. handling the preparation of the address details array, getting the 'created_by' etc). And make your addSupplier method work with a set of arguments that could be run from anywhere in your app.

public function addSupplier ($primary_contact_details, $contact_addresses, $created_by = NULL) {
    // ADDING THE SUPPLIER BASIC INFO
    $supplier_fillable_values = array_merge(
        $primary_contact_details,
        ['created_by' =&gt; $created_by]
    );
    $new_supplier = Supplier::create($supplier_fillable_values);

    // ADDING THE SUPPLIER ADDRESSES AND THE ADDRESS CONTACTS
    foreach ($contact_addresses as $address) {
        // ADD ADDRESS
        $new_supplier_address = $new_supplier -&gt; addresses() -&gt; create([
            'address'       =&gt; $address['address'],
            'created_by'    =&gt; $this -&gt; getAuthUserId()
        ]);

        // ADD ADDRESS CONTACTS
        foreach ($address['contacts'] as $contact)
            $new_supplier_address -&gt; contacts() -&gt; create([
                'name'          =&gt; $contact['name'],
                'phone_number'  =&gt; $contact['phone_number'],
                'created_by'    =&gt; $this -&gt; getAuthUserId()
            ]);
    }

    return $request -&gt; address_contact_inputs;
}


Secondly, it's good practise to avoid too many levels of indention, so you could  refactor to use one level of indentation and splitting out the foreach loops to their own public/private methods on the same class.