How are marijuana laws enforced from state to state even if it says it's not decriminalized or legal

As my AP gov teacher would say, it depends. But ultimately, non-medical possession of marijuana is still considered illegal by the national government, although the individual state can decide whether it is legal or not. In terms of your question, the national government regulates inter-state commerce. Buuut states are usually the ones who enforce rules on possessing marijuana... so it depends on the law of the specific state you are in the boundaries of, more or less.