As you said, you admission is confirmed in a university in the United States. If they are starting with online classes, it is done for all the students across different regions in the world. Due to the ongoing pandemic, no body knows for sure when the situation will improve. This is the reason why your university is not saying anything about physical classes. When people migrate from one place to another, they come in contact with several people and risk of infection increases.

Any way, you should start with your online classes. The university will notify you about the commencement of physical classed beforehand.