I am not a biologist, but what about calories that just go through your body without being absorbed? Or maybe there are calories that do not turn into get but

I am not a biologist, but what about calories that just go through your body without being absorbed? Or maybe there are calories that do not turn into get but muscle? You still getting weight, but good weight? Of course, if you need x calories and get less that x, your body will need to burn something. Maybe brain or muscles and not fat? Again, I do not know much about that but it just may be more complicated than just cals in vs cals out.