My tip for connecting the combo pipes is to connect them to the middle of an existing pipe or road. Trying to connect the pipes end-to-end is very difficult. The snapping doesn’t work very well.
After connecting, you can go into underground view and delete the extra pipe ends if you don’t like them sticking out.
For connecting sewage outflows, there is a small brown dot that needs to be connected to the pipe network. Zoom in to make it easier to see.