what's the difference between <hr and <p ?

Answer 1

<hr />
Creates a visual horizontal line/rule to begin a new line.

<p> </p>
Creates a new line with or without content inside the begin and closing tag.

They are both newline tags. Therefore both will create a new line when the tag is used.
https://www.w3schools.com/html/html_paragraphs.asp
https://www.w3schools.com/tags/tag_hr.asp

Answer 2

By the way, the "attributes" tag for the post is a bit misleading. Horizontal rule and paragraph are elements.