Connecting holes

Hello, I’m trying to connect some channels/holes at 90° inside a wall of this item i am designing, but I cannot get them to connect or run into each other. Please could someone help?

I will also on another part make multiple square holes that are 0.5X0.5mm that are spaced together forming a square of a mesh-like texture. How do i make this easily and spaced proportionally to each other?