HIIT combined with strength training leads to burning calories beyond the time you are working out. You are simply using different energy systems in your body for the two types of workouts. Whereas strength training builds muscle mass, HIIT works on the heart rate. The underlying explanation is of course the elevated heart rate as you go through different exercises at a rapid pace.
If you only do cardio exercises, you will burn both fat and muscle. Also if you do too much cardiovascular workouts, you may find that the metabolism slows down.
Consider using compound movements which are multi-joint movements. These work several muscle groups at a time. One example is the squat exercise, which works to engage both the lower body and the core.
With these insights at hand, you are well on your way to take your knowledge of fitness and how to optimize your fitness regimen to suit your goals!