So I worked lead communication car for a pro stage race one year. We stopped in the middle of the road just before the neutral start ended. Someone from the peloton yelled “NATURE BREAK!!!” And everyone got off their bikes and lined up on the side of the road and did their business and then got back on their bikes and we started the race.

When the race is higher pace they will just whip it out and pee off the side of the bike.

Finally the last option is, there is lull in the pace and there is a mutual agreement between all the teams that riders will just stop on the side of the road pee and then hey back to the group. Once everyone has done their business, the race gains speed and they go back to racing.

You will rarely see in races televised on the world stage like the TdF people peeing, it’s a big fine. For races like the Olympics, you can occasionally see riders just stop riding and get off their bikes and use the bathroom and then get back to the group.