He is using WBTC on the Binance smart chain BEP20 which is not Bitcoin, it is an altcoin.
Sending WBTC to another exchange is a way to minimize transaction fees. Since it is in BEP20, the user can minimize the transaction fee
There's no such WBTC on BSC network since WBTC is a dedicated token that run in ETH network. If you use Bitcoin that run in BSC network, it's actually
BTCB.
but if I what to chose the first bitcoin network I will also chose same in my mexc if not it will be sent to a wrong address despite its still bitcoin. so changing its network automatically turns the bitcoin to wBTC.
Remember, both Bitcoin and BTC(SegWit) are actually same, segwit is a soft fork and it's compatible with the original Bitcoin chain. So don't get tricked by Binance since they make it separate.