Let me start by saying; that it's not important to know how CatDog go to the bathroom. HOWEVER, this video is very well put together! Of course, cartoons aren't meant to be logical all the time, and it's ok to have some unanswered questions. I saw this video pop up when checking for new CatDog videos and didn't want to watch it because I assumed it was another dumb "how do they go to the bathroom??" thing. But I decided to give it a watch because I currently have nothing else to do. This video really explains a lot and actually got me thinking. While just because something is mentioned for the plot doesn't mean it's canon. Such as Dog lifting his leg to the golden hydrant. I like to think they use the bathroom through their armpits and never really gave it a serious thought. But perhaps it could be a "teamwork" process between them and they go in the middle. We'll never know a "canon" answer, but that also makes it interesting because you can decide how you think they go and it adds to the creativity of CatDog. But again, this is not something that's important. I can't stand that question being what most people associate with CatDog. It's always "how do they go to the bathroom" and never "CatDog is a great show" or "They're adorable, love this show, etc" It's ok to not know.
https://www.youtube.com/watch?v=qBlxnj0uSIg