Movies from Hollywood are usually released here in the US first, or at least at the same time in theaters. This is similar with a company starting first in their home country with a product or store before going to other countries. But occasionally the opposite happens.
Avengers: Endgame opened in China before the US. It opened a couple of days earlier there. In fact, it had over $100 Million in China at the box office (not the studios take) before it opened in the States.
To go international first is unusual but to add to that in China is even more strange. China is a huge market that can show a lot of promise.
What made Marvel choose this route? Here are a few factors I believe were considerations. First, piracy, it is a problem in China, like everywhere else but can be even more so. With the fear of such a big movie being leaked from western releases they might have wanted to get a jump on the problem. Also, they may have wanted to give it a bit of an advantage because of recent releases from Lucasfilm (another Disney owned company) Solo, which came from the huge franchise of the Star Wars universe. It didn't do as well in China as they hoped. It bombed everywhere but China was one place they thought might save them in Solo's case. Last, I believe they wanted to also get ahead of the Chinese Labor day (May Day) as movies are relesed on Wednesdays in China. Since May Day is Monday April 29-Wednesday May 1st, they probably wanted to jump into the week before and get as much of those days off as possible when it came around the next week.