Get product collection by category id on phtml file magento2

You can add this in the Block, and call the function getProductCollection() in phtml,

protected $_productCollectionFactory;
protected $_categoryFactory;

public function __construct(
    \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productCollectionFactory,
    \Magento\Catalog\Model\CategoryFactory $categoryFactory,
    \Magento\Framework\View\Element\Template\Context $context
) {
    $this->_categoryFactory = $categoryFactory;
    $this->_productCollectionFactory = $productCollectionFactory;
}

public function getProductCollection()
{
    $categoryId = 'yourcategoryid';
    $category = $this->_categoryFactory->create()->load($categoryId);
    $collection = $this->_productCollectionFactory->create();
    $collection->addAttributeToSelect('*');
    $collection->addCategoryFilter($category);
    $collection->addAttributeToFilter('visibility', \Magento\Catalog\Model\Product\Visibility::VISIBILITY_BOTH);
    $collection->addAttributeToFilter('status',\Magento\Catalog\Model\Product\Attribute\Source\Status::STATUS_ENABLED);
    return $collection;
}

you can simply call block and get product collection using below code,

Inside BLock file,

<?php
namespace Company\Categories\Block;

class Categoryproduct 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;
    }
}

call function in your .phtml file,

$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>