How could I build related post section like this?

Hi there,

That site seems to be using a plugin named "Contextual Related Posts" for the post list, hooked to generate_after_content. :)

Not exactly sure which way he hooked it in but it can easily be done w/ either Hook Element, Block Element or manually through code using add_action().

Yeah thank Elvin so much ^^ I got the concept, will try later :D

Nice one. No problem. Let us know if you need further help. :D