1) Area effect cloud spawn when a player dies.
The area effect cloud will have the exact same potion effects as the player who have died which will then transfer to nearby players.
2) Collectible Area Effect Cloud, by simply throwing an empty bottle into the area effect cloud you'll get a potion bottle with an effect from that cloud but the duration of that potion will be vary depending on how long has that area effect cloud exists.
Do note that if area effect cloud consists of multiple potion effects the bottle will only pick one potion effect and leave others effect still inside the area effect cloud.
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And also thanks Chuckchuk/Axel for helping me create those gifs.