I've seen this question quite a lot and i asked the same question from my friend, answer is very easy.
You don't have to download anything, all you need is retail copy of ArmA3 PC game, then simply launch the game, go into server browser / multiplayer and look for a server that is running "Wasteland" or "Altis Life" and then just join that server, game will automatically download the mission file/s which is 1-5mb in size, it will not effect your ArmA 3 in any way, you will still be able to play in any other server.
In other words, Wasteland and Altis Life are simply missions and not mods, it is completely server sided and you don't need any additional files, just join a server and you're done!
Hope this helps you people out there who got confused about Wasteland and Alits Life, like i did before i tried them out :)
That said, here's a playlist of my ArmA 3 Gameplays / Stories from Wasteland and Altis Life
Gaming, Hardware Reviews, Crash Compilations and more :-)