Help in converting vanilla PHP to CI (AJAX + filter + infinity scroll)

PHP Code:

<?php
defined('BASEPATH') OR exit('No direct script access allowed');

class Json extends CI_Controller {
    
    public function return_json()
    {
        if( !$this->input->is_ajax_request() )
        {
            exit;
        }
        
        $json = $this->input->raw_input_stream;
        $json = json_decode($json);
        /**
        $json now looks like
            stdClass Object
            (
                [group_no] => 1
                [brand] => brand
                [material] => material
                [size] => size
            )
            
        Access it like
        echo $json->group_no;
        **/
        
        // Process json...
        
        // Data to send back
        $data = array('success' => 'true');
        
        // Send back json
        $this->output
                ->set_content_type('application/json')
                ->set_output(json_encode($data));
    }
    
    public function return_html()
    {
        if( !$this->input->is_ajax_request() )
        {
            exit;
        }
        
        $json = $this->input->raw_input_stream;
        $json = json_decode($json);
        // Process json...
        
        // Data to send back
        echo '<article class="col-md-4 col-sm-6">...</article>';
    }
} 


Code:

$.ajax({
    url : "/json/return_json/",
    type: "POST",
    data: JSON.stringify(
        {group_no: 1, brand: 'brand', material: 'material', size: 'size'}
    ),
    contentType: "application/json; charset=utf-8",
    dataType   : "json",
    success    : function( returnedJson ){
        console.log("Pure jQuery Pure JS object");
        console.log( returnedJson );
    }
});

$.ajax({
    url : "/json/return_html/",
    type: "POST",
    data: JSON.stringify(
        {group_no: 1, brand: 'brand', material: 'material', size: 'size'}
    ),
    contentType: "application/json; charset=utf-8",
    dataType   : "html",
    success    : function( returnedHtml ){
        console.log("Normal html");
        console.log( returnedHtml );
    }
});