Well, if you've got to ask the question, maybe you already guessed. Yet, there are several options.
The hardest thing is avoiding filling up your socks, because then it will get into your shoe (usually right shoe) and squish, squish, squish. . .breaking your concentration very, very well.
Sometimes they do get to pull over and pee. You know those TV spots where the majority of the riders appear to be loafing, like your grandma could outrun them? This was filmed about the same time as a mass "nature call". They just don't film the guys peeing.
This option does not exist in the time trials.
Those who have the knack of drinking about as much water as they sweat and breathe out don't have to stop. The guys with squishy shoes are the ones who drink a lot of water as a good way to stay cool.
In an emergency, you can also get a bit sideways on the saddle, jam an empty, lidless water bottle up the leg of your shorts and just cut loose. That works really well if you've got extraordinary balance.
Technically, if you're racing back to back for many days in a row, you have to drink a lot of water in order to avoid losing muscle. 1 water bottle for every 25-30 miles isn't good enough in back-to-back racing. And in this case, you can guess what happens.
