Much less?

A has £100,000.
B has £95,000
C has £3,500.

Be has less money than A. C has much less money than A or B.

Thank you. I understand that. But what does it mean in this sentence 'The town I grew up in doesn't even have a grocery store, much less a shopping mall'.

That sentence is saying,
"The town I grew up in wasn't large enough to have a grocery store, so there was definitely no room for a shopping mall."

If it helps, you can think of 'much less a...' as a shortening of, 'so it was much less likely there was a...'

vts nair 'The town I grew up in doesn't even have a grocery store, much less a shopping mall'.

"much less" is an idiom meaning that the existence of "a shopping mall" is a preposterous idea, given that there was not even a grocery store.

She'd never even held a gun before, much less shot one. What does it mean here? She didn't hold a gun before. So certainly she don't know how to make a shot with it?. Is my interpretation right?

Yes, it's spot on!
Also, she certainly wouldn't know how to shoot a gun.

Oops!
I'm wrong there.

She wouldn't know how to fire a gun
or
Shoot with a gun.

Yes.

She'd never even held a gun before, much less shot one.

She never had any experience with a gun. She never had one in her hand. It would be absurd to think that she had ever shot a gun.

It does not tell us that she would not be able to shoot a gun. Maybe she watched people shooting.

Yes, that's much more to the point.