Passive income is a process of having an income without such work. This includes investments in bitcoins, ethereum, altcoins, real estate, mutual funds, bonds, etc. Mining, in my opinion is included because you just buy a hard drive and let it mine for you. Generating passive income means that you must invest in any of this either 1 or more.
Yes, you right. If we want to maximalize passive income we must keep focus on growing asset. Bitcoin is good for begining, but if I have big capital, I prefer like to do more by create or buy home, store or even real estate. I will rent those asset.