Pacemaker implantation surgery question. where do they poke the wires through to get them inside the heart?

The veins.: The veins under the skin in the upper chest are accessed to place the pacemaker/icd leads. These are the axillary or cephalic veins. The leads are then guided by xray and advanced into the heart chambers.

Under the Collarbone: After we open the skin and tissue, we use needles to access a vein called the axillary vein that runs just below the collarbone. There are two other ways to do it, but that's the safest and least traumatic to the surrounding tissues, generally speaking.