Nothing specific, what are the settings behind that when you scroll down the page, more posts are loaded?
Is this enabled by default in the Theme?
Not sure what you mean by this question.
That’s already what AJAX pagination does. It loads more post depending on the query.
Example: If the query specified has 47 posts and the post list has post per page of 10, the initial display will list 10 posts and then loads another 10 posts every time you reach the bottom of the post (or press the load more button). And since the query isn’t exactly divisible by 10, the loading would be 10 > 10 > 10 > 10 > 7.
AJAX pagination being disabled/enabled depends on which adds it in. If the query was from a plugin, then the plugin enables it. If it’s from a theme, then the settings is likely on the customizer settings.