his bodily functions do not magically speed up or slowdown if someone is traveling in a fast vehicle, does that even make sense?? Of course not
The perspective and experience one has always seems to be the normal, correct one.
It's how your time dilation is RELATIVE to others moving at much different speeds that matters.
Think about using your fast car example. ( I've actually used this example in class).
You and I are driving down the road , seated inside a car at 60 mph.
Relative to each other we never change position. We can talk, listen to music, eat, drunk, etc.
A person standing on the side of the road observes us drive by.
From his perspective, we are a blur. From our perspective he's a blur.
Believe it or not, time moves differently depending on your motion thru space.
Relativity isn't my area of expertise and I don't claim to fully understand the details of all it's equations and effects.
Most people can only comprehend things in their own world. Newton felt time was an endless progression that never changed.
He was an incredible physics genius BUT he was wrong about time.
I'd invite you to google how GPS works with relativistic effects and that might help.
You seem to have a good mind. I'm not sure about your background, but you
might consider enrolling in a graduate or undergrad physics degree program.
To get full comprehension you need to get into the tedious details you won't get from lighter,"fun" sources>