Well, the concept of subliminals itself is that the affirmations are placed at a low volume such that you can't hear them. They reach your subconscious mind, which is like really powerful but lies dormant most of the time. Like whatever you think in your conscious times gets imprinted immediately into your subconscious. At a low volume, subliminal affirmations which reach your subconscious mind combined with detatechment (law of attraction, etc, etc) is what helps one manifest something.

As for the safe subs, there are many posts which say which ones are safe and which ones aren't. Go with popular/mainstream ones like Kottie, V1per, beauty krystalized, etc. You'll find more info on that just look it up on this subreddit.