Most substances have a unique psychoactive effect based on the receptors in the brain they attach to.
Alcohol is a central nervous system depressant that lowers inhibitions and impairs speech and coordination.
The THC in marijuana attaches to cannabinoid receptors in the brain that cause euphoria and distortions of perception. When mixed with alcohol, it can amplify THC levels and boost the effects of each substance.
Opioids are a central nervous system depressant like alcohol. Bothe substances can cause cognitive impairment and reduce respiratory function, so using them together boosts the depressant qualities of each. This can be dangerous and may lead to respiratory arrest.
Cocaine is a stimulant that by itself increases energy, mood, heart rate, and blood pressure. But when cocaine is cross-faded with alcohol, it produces a metabolite known as cocaethylene that intensifies the stimulating effects. This can induce thoughts of rage, anger, and unhealthy behavior.
https://en.wikipedia.org/wiki/Cocaethylene