both. nuclear powered submarines are the primary wing of nuclear deterrent because launching a missile from a platform that stays underwater for months at a time and is basically impossible to track is basically impossible to prevent

I have not read that the US is giving SK the technology to build nuclear-armed subs - that is, ballistic missile submarines - but only nuclear-powered subs.

The US has those and also nuclear-powered but conventionally armed subs. The ballistic missile subs hide and, if called upon, launch their nuclear-armed missiles (which never has happened, of course). The conventionally armed subs attack ships and other subs.