I generally consider skill work to consist of exercises that are difficult for reasons of mainly coordination or balance. These exercises are not static, and will change as you progress. For example, freestanding handstand push ups may be skill work for a long time. But, with enough dedication, you can get to doing consistent sets of 5+ reps, and you can use it as a strength exercise in a regular workout.
Skill work isn’t meant to drain your physical strength, it’s meant to fry your nervous system. You could work on your handstand for like 5 or 10 minutes and then feel it’s much more difficult to hold the handstand, yet you still feel like you could do about as many push ups as you could before doing the handstands.
However, skill work can still involve a strength component. Most advanced calisthenics are highly compound. Being able to hold a proper back lever and track your body in space while maintaining the full body tension is a skill unto itself. Yet this is a skill that requires time in the back lever, so it is developed continually as you practice the lever.