Back in the day I was a member of a Discord where people were constantly giving away keys. Most of the games were AAA. The problem was that it was really hard to be on time, because one second after someone shared a key, some other member had already grabbed it. I never managed to get any that way.
I know there are sites that run promotions and give out keys too. There’s also Epic, which gives out good keys from time to time, but I specifically wanted them for my Steam account. Especially for the games No Man’s Sky and DayZ.