They have to learn that they can buy a fraction of BTC like 0.005BTC, I think this is the most basic and they can send BTC to anyone as long as the wallet destination is also a BTC address to which exchanges also have BTC address where they can deposit. This is the most basic and no blockchain knowledge is needed.

Know when to buy and when to sell. This I think is the only I want a newbie to understand as an investor because it's the only they can make money in investing BTC.
This includes learning when it's the halving and then the bear market will almost be over. Trading BTC is a different topic for a newbie.