How did David Bowie manage not to die in 1975/1976 when he was very addicted to drugs and alcohol?

He had youth on his side but the life style will always catch up with you. Simple cause and effect.

You could ask the same about Lou Reed, Kieth Richards, Iggy Pop, or anybody in Motley Crüe in their young years. Perhaps when it's your time, it's your time, who knows?

I really don't know much information about David Bowie

Who knows, i am always amazed that keith richards is still alive.

Well, it's hard to say, and he smoked cigarettes one after each other too,

Or as we say a chain smoker.

He probably didn't OD (Overdose).

He knew just how much stimulants he needed. Many artists from that period knew their limits. Look at Keith Richards--he survived. Many artists survived heroin and coke addictions. Then you have those who drank to excess and died. Some artists just did drugs more carefully.

he was hanging it out for the American HIV virus and AIDS

You'd be amazed at how difficult it is for some of those people to die. The build up a super-high tolerance level over time.