Barky , first you have to know guitars in general.
In a nutshell ;
The standard six string guitar has strings tuned ( low to high ) E,A,D,G,B,E.
The stardard four string bass has those lowest four strings, E,A,D,G
Now…for the five string bass, we add a string with notes LOWER than the low E. It has a low B string.
( there is also a six string bass tuned B to C or any other way the player wants )
May not mean much to you without general stringed instrument knowledge.
SO…
These instrument facts relate to cars how ?
Much the same as us car guys talking gear ratio !
It means a lot when put into perpective to the media.
When discussing the difference between a bass guitar and a 6-string it would help to understand octaves too.
Regarding the “one string for each finger” theory, it doesn’t work like that at all. There are a total of seven major notes, and four flats/sharps. When you hit the eighth major note, you’ve gone up an “octave”. Think eights as in octagon or octopus. The guitar is arranged such that the eight major note will vibrate at twice the frequency of the first note, and every multiple of a note’s frequency is the same note only one octave higher. A six string guitar has a total range of just under four octaves. The sequence travel all the way up the fretboard on each string, repeating itself when it hits the twelfth fret.
Each individual string has a total range of just under two octaves. Each note can be played in numerous places on the neck. Each major chord contains three notes, any combination of the three notes creates the desired chord, even if there are multiples of the same note in different octaves being played in the chord formation, and the six-string guitar enables a number of different ways to play a desired chord, enabling great flexibility.
For example, a “C” chord is comprised of a C, a G, and an E note. Each note has multiple locations on the fretboard. Any way you can find to play those three notes together, regardless of the octave of each note and regardless if your formation plays one of the notes in two places at once, that will comprise a “C” chord. There are a number of ways to do this, because each note has numerous locations on the fretboard.
Those are the raw basics. I won’t go any deeper, 'cause by the time you start thinking about adding more notes than three to a chord it gets more complicated and you need to understand what you’re doing.
Oh, I almost forgot, there is one rock guitarist who has custom made guitars with seven strings. I’m not sure which end he adds too, the low end or the high end. I can’t imagine he’d add to the high end 'cause I don’t thing there’s a string that’ll go there.
Sincere apologies for getting off subject. I’ll try to do better in future. 
Yah, that’s like trying to ‘splain gear ratios to the kids.
’‘Why does the trasmission do that shifting thing ?’’
( keeping it automotive )
and…
You can buy a seven string, not custom made. The most common addition is the low B string but many players custom tune. This is one reson why you’ll see players change guitars. Not only for different sounds or re-tuning back-stage but for one custom tuned ( and a hidden reason why some songs are so dang hard to learn for us weekenders learning by ear. )
But at the very rudements, the basic triad chord is the first, third, and fifth asT.S.M. describes in C, In any given scale ( think ; doh-re-mi-fa-sol-la-ti-doh ).
Yup, retuning is a good way to change keys.
Never saw a seven string that wasn’t custom… but, then again, I never looked for one!
But both concepts get complicated to someone struggling with the basics.
It always bothered me that when many guitar teachers teach the “scale” they teach the notes all at the nut end of the guitar and starting with the low E and on to the G or A on the first string. That isn’t really the scale. It’s the scale twice over (over two octaves) with a few notes added at the high end. The scale goes all the way up the neck on every string, over and over, in a fixed pattern. It isn’t just located between the nut and the third fret.
Oh well, I guess that’s why I only took two lessons!