Please try below code to get the first level category collection in your template phtml file:

<?php
    
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();

    //get current category
    $collection = $objectManager->get('\Magento\Catalog\Model\ResourceModel\Category\CollectionFactory')->create();
    $collection->addAttributeToFilter('level', array('eq' => 2));

    foreach ($collection as $category) {        
        print_r($category->getData());
    }
?>

Note: Direct use of objectManager is not recommended. Try to create the function in your block class and use it in the template.

Or you can get the Parent Categories and check the level in for loop!

use \Magento\Framework\Registry $registry
...

public function getCategoryCollection(){

        $currentCategory = $this->_registry->registry('current_category');
        if($currentCategory){
            if($currentCategory->getParentCategories()){
                foreach ($currentCategory->getParentCategories() as $parent) {

                    //check level
                    if ($parent->getLevel() == 1) {
                        return $parent->getId();
                    }
                }
            }
        }
        return null;
    }

Hope this helps!

Please try below code to get the first level category collection in your template phtml file:

<?php
    
    $objectManager = \Magento\Framework\App\ObjectManager::getInstance();

    //get current category
    $collection = $objectManager->get('\Magento\Catalog\Model\ResourceModel\Category\CollectionFactory')->create();
    $collection->addAttributeToFilter('level', array('eq' => 2));

    foreach ($collection as $category) {        
        print_r($category->getData());
    }
?>

Note: Direct use of objectManager is not recommended. Try to create the function in your block class and use it in the template.

Also, you can get the Parent Categories and check the level in for loop!

use \Magento\Framework\Registry $registry
...

public function getCategoryCollection(){

        $currentCategory = $this->_registry->registry('current_category');
        if($currentCategory){
            if($currentCategory->getParentCategories()){
                foreach ($currentCategory->getParentCategories() as $parent) {

                    //check level
                    if ($parent->getLevel() == 1) {
                        return $parent->getId();
                    }
                }
            }
        }
        return null;
    }

Or try to inject the \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory class in your block class

use  \Magento\Catalog\Model\ResourceModel\Category\CollectionFactory $categoryCollection

public function getCategoryCollection()
    {   
        // $level = 2;
        $collection = $this->_categoryCollection->create()
            ->addAttributeToSelect('*')
            // ->setStore($this->_storeManager->getStore())
            ->addAttributeToFilter('level', array('eq' => 2));
            // ->addLevelFilter($level); OR
            ->addAttributeToFilter('is_active','1');

       return $collection;
    }

Hope this helps!