Bank VS broker

Hi, I (21m) just started working after graduating this year. I want to start investing 125€/month but don’t know to do it trough the KBC app or through BUX zero. I want to buy the S&P 500 on broker, or the biggest option on the KBC app.

I only start investing a small amount, because I am saving up all the rest so that I can buy a house in 4/5 years.

Thanks in advance