How to get products collection by category id in custom module

Create a block call getProductCollection function.

<?php
/**
 * Abm
 *
 * Author : Tony Liu
 * Blog: https://www.tonyblog.cn
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade this extension to newer
 * version in the future.
 *
 * @package Tony_GetProductsByCategoryId
 */
namespace Tony\GetProductsByCategoryId\Block;
class GetProducts extends \Magento\Framework\View\Element\Template
{
    protected $categoryFactory;
    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Catalog\Model\CategoryFactory $categoryFactory
    ) {
        $this->categoryFactory = $categoryFactory;
        parent::__construct($context);
    }
    public function getCategoryProduct($categoryId)
    {
        $category = $this->categoryFactory->create()->load($categoryId)->getProductCollection()->addAttributeToSelect('*');
        return $category;
    }
}

Show products in template.

<?php
/**
 * @var $block \Tony\GetProductsByCategoryId\Block\GetProducts
 */
$categoryId = 5;
$getProudctcollection = $block->getCategoryProduct($categoryId);
?>
<ul class="category-products">
    <?php foreach ($getProudctcollection as $product) : ?>
        <li class="level0-child">
            <a href="<?php echo $product->getProductUrl(); ?>">
                <?php echo $product->getName();?>
            </a>
        </li>
    <?php endforeach;?>
</ul>

If done this, you will see products of one category.

If you need full package, please go my github.
https://github.com/tonyabm/magento2-module-GetProductsByCategoryId