Recently, I heard that China banned holding cryptocurrency and cryptocurrency transactions, so does that mean that having cryptocurrency in China would be a criminal offense? Moreover, why create such a ban in the first place?
First of all, the simple question is, if the state forbids something in its constitution, you cannot do it, or you can, but you will bear sanctions because you violate the law.
Cryptocurrencies aren't banned in China, at least for the time being, which doesn't mean it can't be changed. If it is forbidden and you continue to use cryptocurrencies, then you are engaging in crime, which the state will not allow.