Link to other portfolio items from portfolio item page

Hello,

I’m trying to get a way to have links to other portfolio items on any single portfolio item page.
Actually, just like on the demo portfolio-item page :
https://demos.cpothemes.com/allegiant/portfolio-item/son/

Does anyone know how to showcase the portfolio in a portfolio item page ?

Thank you so much in advance, I’m struggling with this for a few days and trying to get into source code but getting confused with php that I’m not familiar with.

note: I use a child theme

The page I need help with: [log in to see the link]