How can the government discover I own BTC, let's say I'm from Venezuela where crypto is banned, what they can do to discover and arrest me?
if the government does not allow bitcoin to be accessed, there is certainly a strict sanction and certainly the government can track that possibility, but if it does not prohibit, the government cannot do anything.
Yeah keep in mind your government's access to information as well. The US government can find almost everything it wants because it's in Google and Facebook's pockets, but most other governments will have a much harder time getting that kind of info out of an exchange, or even finding out that you are a part of that exchange in the first place.
Now the KYC process enables these exchanges to give over that info, but they don't necessarily do it for everyone. Only the ones who are big problems.
And like said above, if you are careful about your conversions (cash only or offshore accounts) then no one will know where it came from.