I know you lift heavy with legs, and light with arms, but when i do excercises with light weight, such as dumb bell bench, or bicep curls, or tricep extensions/pushdowns, it dosent feel like its even doing anything. Do I have to do them slowly or something?

“Light” is a relative term. If you can do more than 15 reps without fatigue, then the weight is too light. If you get muscle failure before 15, than the weight is too heavy. Pick a rep range (15 is the max) and follow that type of logic. If you can do more than 15, add more weight for the next set (or take weight off if you do less than 15). You should see progressive gains in your strength.

But, to be honest I have never seen a problem with lifting heavy on lower and upper body. As long as you lift with a goal in mind (speed or explosion) you will create muscle to do just that. Maybe others could shed light on their techniques.

Well, I dont want to lift heavy with upperbody because that will add more muscle, which will slow down my velocity.

you can bulk up a bit in the upper body, just not to much so that it restricts your range of motion