I'm having a real headache understanding something very simply. SI units for resistivity.

Here's the link to the Wikipedia page: Resistivity

Now the thing is I can understand the resistivity equation and understand it's proportional to length and area, however the bit I just can't grasp is the ohm-meter unit as it's not ohm per meter but seemingly more ohm per area. I have a fluid whose resistivity is stated as 1.5Mohm.m, however that fluid is tracking across a gap of 1mm.

According to online calculators, 1 ohm.m = 1000 ohm.mm. 1.5Mohm.m = 1.5e+9 ohm.mm.

How is that division working? I just can't get it straight in my head other than a smaller area means higher resistance.

The other question I have is to do with unit affixes. Take these images as examples:

What does the -1 above each of these mean physically and mathematically? Is it meant to show the reciprocal of the actual number? These aren't powers or exponents, so what do they represent?

Cheers for looking and thanks in advance.