I have a question that's been bothering me for a while. I know that Naruto is supposed to be a manga written for teens and pre-teens so some stuff doesn't have any explanation or represents a plothole, but still I would really like to know whether this question has a reasonable answer or not. How can Obito transport his body parts in another dimension without the person that passes through him getting blood over his/her entire body? And why doesn't the body just collapse with the head being in another dimension?
Obito uses a technique called Kamui.
Kamui is a powerful Mangekyō Sharingan dōjutsu that creates a unique and specialised form of space–time ninjutsu. It allows the user to achieve two very distinct, yet closely associated feats — teleportation and intangibility.
Intangibility, here refers to making the body or certain parts of body inaccessible to the physical world. Thus, no one could touch him and passed right thought Obito.
This is just my own theory, but it might be like in UQ holder where the vampire's arm, though sliced off, is still connected to its body through magic (here chakra) links, meaning that blood and nerve signals reaching the "hole part" are teleported to the other dimension and same upon exiting the hole part.
This would explain why he feels the pain of being stabbed in the kamui dimension while his head is in the normal one
Basically, according to the body system, it is still whole, and it can function perfectly through dimensions