How to get current post user id

To show author avatar per post inside loop you need to modify a little:

$current_post_author_id = get_the_author_meta( 'ID' );
<?php if (!empty(get_user_meta($current_post_author_id, 'user_avatar', true))): ?>
<img src="https://wordpress.stackexchange.com/questions/317789/<?php echo get_user_meta($current_post_author_id,"user_avatar', true); ?>" alt="some text">

Try, if it works for you.