Magento – Registry deprecated in magento 2.3, How to set data without Registry

Take a look at /app/code/core/Mage/Adminhtml/controllers/Sales/OrderController.php

/**
 * Initialize order model instance
 *
 * @return Mage_Sales_Model_Order || false
 */
protected function _initOrder()
{
    ....

    Mage::register('sales_order', $order);
    Mage::register('current_order', $order);
    return $order;
}

In order to use Mage::registry('current_order') it must first be set (Mage::register('current_order', $order);) some where , else you going to get a Fatal error

The registry stores data to memory which is specific to that request (rather than user or anything else), and persists for the duration of that request only. The principle is very simple really, the Mage class is instantiated as a singleton object for every request and the instantiated Mage object remains in memory, and is accessible in all classes (and templates) until the request completes and the response is sent.

As the object is a singleton, whenever you access it you get the same object. All that is happening is that you are storing values to this object, so when one class stores a value, and another accesses it they are both working on the same object and the second class is able to retrieve the value the first class set.