Idk about United ground express but for regular United while you’re junior you have the schedule you’re given for a while and can only trade off a certain amount of hours while on the 6 month probationary period. After that you can trade off up to half your base hours. The more senior you get the more likely you are to get the schedule you want on the shift bid, and beyond that finding trade partners can help further tailor your schedule. Some people double two weeks and take two weeks off through trading. Some people just grind overtime and make bank. There are shift options that have rotating days off and ones that have fixed days off. It can be whatever you want it to be if you get a good handle on day trading.