I've also wondered about this, but it seems as though there isn't only one way to writing a comic script, just as long as you describe the events/actions and make who is speaking in the panel clear. When I write scripts for myself, it's easier for me to distinguish where I separate my panels (different paragraphs) and where I have a separate speech bubble (each on a separate line, even if it is the same speaker). If you are writing for someone else, you would have to provide more details.
Some threads that have also discussed this that I refer to are:
And also, https://www.makingcomics.com/2014/03/08/write-script-comic/