WP-Model-virtual.php

<?php
Class Product{
    ...
    public $virtual = [
        'humanWeight',
        'seller',
    ];

    public function _getHumanWeight(){  
        return $this->weight . 'Kg';
    }    
    
    public function _getSeller(){
        return Seller::find($this->seller_id);
    }
}
$product = Product::find(10);
$productWeight = $product->humanWeight; // '10Kg'
$productSellerName = $product->seller->full_name; // 'John Doe'