If you contract mucles as if you were lifting a heavy weight, is effect the same workout wise?

No. While contracting the muscles does work them, using weights (especially really pushing it) creates tiny tears in the muscle tissue. Those tears healing is what "builds" muscle and makes it bigger and/or stronger. You can get some progress with body weight exercises, but to continue building muscle, you have to add more weight and really push it/challenge it. Just contracting it does not have the same effect (or gyms would go out of business). Nobody would go there and use machines or lift weights, if they could get the same result staying home and simply contracting the various muscles.

Its called isotonic exercise, it does work but is not as effective as isometric lifting weights.

No, different muscles are activated with weights and without. And actually you are not damaging your muscles without weights so effect would be different or no effect at all. To not say the worse, you ll practice to control your heart and not the help to the heart and may result in bad combination for your future of health and survival, so that is why are relaxing words and time for relaxing invented for any part - meaning never say "mind is", but "body is" and even better say "can you eat my hgh?".