I'm converting the upstairs of a garage to an apartment. There is no plumbing now and I'm installing a septic for the apartment. Can I put the main drain to the septic on the exterior? Otherwise I have to punch through the garage concrete slab?

Yes, you can install main sewer on the exterior wall. Use Cast Iron where pipe is exposed. You could also use ABS or PVC but in this case you will have to box them in.