Sorry if this question is asked several times, but I'm lost finding a place to buy some crypto currency.

Is the following website fine for example: https://buy.bitcoin.com/

After buying some BTC from that website, can I do whatever I want with that amount? For example, I want to buy ETH or invest in other coins, is this possible?

For example, on www.bitcoin.com, I also found this website: https://cex.io/buysell. Any opinion on this one? What's the difference?

What I don't understand is, why there are so many websites where I can buy crypto currency. It's hard to find one which is qualitative and meets the requirements... I want one where I can buy other cryptos with the currency I have at that moment...

One more found... what about this one? https://shapeshift.io