Is the Republican Senate Runoff closed or opened? What are the voting rules based on party?

I know the primaries that took place this week are open. Do the same rules apply for the runoff between Paxton and Coryn?

https://slate.com/news-and-politics/2026/03/texas-senate-republicans-primary-whos-the-favorite.html?fbclid=IwdGRjcAQXMHJleHRuA2FlbQIxMQBzcnRjBmFwcF9pZAo2NjI4NTY4Mzc5AAEe4D-Vhg8vvxw3IW38hj2yZHr5SPs7lfc9U6WFnButJVRgsjLkOYcyYC1xbtA\_aem\_e84GO45kiT02wHLXUhsyug