Thank you guys.
@Thorleyian, I understood everything (I use the translator for the things I wasn’t sure what they mean), you’re being as much critical as I asked for it, so thank you again.
TIG and DaveR gave me a graphical explanation sharing your point about why using components is a better choice.
I realized that I uploaded the wrong .skp file, the “right” one has the middle lines hidden as you suggested and there is no a group for each side, only components but I still have some groups inside certain components.
I attach the .right skp file if someone want to give it some utility it has also a couple of details more, the symbol that looks like a pawn from a chess table on the front and the bumper.
car3.skp (3.0 MB)
Thank you.