Comfy, as in>reminds me of pleasant things so i dont have to feel bad about my worthless life
or>does not contain things which remind me of my worthless life, like adult struggles, emotional difficulties, responsibility, etc.
Which is why "comfy" is almost always associated with two sets of things:
1) childish, simple things like high school love, friendly interactions with people, animal companions, low-stakes situations, playing games with your friends and having a good time, people telling characters it's okay to enjoy their childish hobbies, etc.
2) being left alone to do what you want, empty towns/spaces, open nature/bright post-apocalypse, fantasy settings where nothing happens, open ended narratives with no actual conflict or story to tell, etc.
Basically, if it falls into one of two categories of (1) let's me pretend to be a kid again or (2) asks absolutely nothing of me and gives me a pretend-space to escape to, then it's "comfy".