To sum it up: I didn't think booleaning. I think it'll probably need to be cleaned up anyway, considering that the joined meshes have faces inside the body. I'm very new to this, which is why I must ask. All I did was the joining of meta meshes by CTRL+J.
Now we have this as a result. How to join this, or is this model doomed? By the way, I think it's worth adding that smoothing at any time will separate the meshes if they are visually but not technically combined (e.g., through the use of the snake hook tool).
In addition, I'd like to add that I've tried booleaning the stuff that I separated with loose parts, but it doesn't work. Things turn out really weird.