UI customization. Sizeble inventory slots

I make my own project using tutorials, and I found an inventory with a UI, as I need, but I don’t know how I can transfer it to my project… I don’t know which variables I need to assign, dont know is there any tutorial how to make it. Please help me
My UI:

The one that needs to be transferred

Link to series with inventory

Nice helping…Amazing community…I cant even close this question…