Chilean passport with valid US visa

I will travelling to South Korea from the US and I will arrive in BEIJING to then travel to HANGZHOU and finally to Jeju South Korea. I understand that I do not need an entry permit if I will be only in transit am I right?

Also, in my way back I will travel from Jeju South Korea to Shanghai where I will have a layover of 19 hours, to then continue my trip to Canada. Will I be able to leave the airport under the 72 hours transit?

Thanks