put together
并 centers on bringing separate things into one group or arrangement: putting together, joining, or lining up side by side.
并 is an ancient character that originally depicted two people standing side by side, conveying the idea of joining or lining up. The modern form is simplified and abstract, but the core meaning of putting together remains.
The top 丷 looks like two small marks being brought together, and the bottom 开 resembles a frame or stand. Picture placing two objects onto a shared tray to put them together.
For ヘイ, imagine putting together a stack of hay bales: hay -> ヘイ, and you are joining them into one pile.