I've learned that when trading (buying or selling) in an exchange, such as Bittrex or Binance, one doesn't get charged with network fees. For example, if I'm selling off some Monero for BTC, I'm not getting charged with the BTC Network fees. This confuses me a bit. Does this mean I never really had Monero or BTC in the first place? And the only time I can have any actual coin is if I withdraw from the exchange? Thanks.