You can do so by creating a child theme and add your unique text to the archive.php page.
See this wonderful guide for more info.
In short, let’s say that the theme you’re currently using is Twenty Twenty-One, you can create a child theme of it and named it Twenty Twenty-One Child.
Create a folder named twentytwentyone-child under the themes folder, and add a style.css file with the following code.
/*
Theme Name: Twenty Twenty-One Child
Template: twentytwentyone
*/
This tells WordPress that this theme is a child theme of twentytwentyone. Now, let’s create a template file to override the file in the parent theme. Since you’re trying to add text in the Category page, the template file we’re going to create is archive.php. (See more on how to know which template files we’re going to use in the above guide)
Copy paste the original archive.php file from your parent theme. Then you can add any text to your page anywhere you like.
In the this specific example, you could try to add the text above the footer.
...
<?php if ( is_category() ) : ?>
<p>Add any text here before footer</p>
<?php endif; ?>
<?php get_footer(); ?>
