>Or how exactly does nudging look like in practice?

If you want people to take the stairs you might make the elevator stay at the top floors so that people always have to press the call elevator button, they might choose to walk up instead of waiting.

If you want people to eat less animal based proteins you can make vegan options the default for catering/meal options with meat only available as a option you can request. Some people will take whatever is the default.

You want more organ donations you make being a organ donor the default with a option to opt-out.

The goal is to make the desired option the easiest way out so most people will avoid the cognitive work of doing what they would actually prefer if they thought about it.