Computer fans

Hello,

I just want to know the correct way to connect the fans inside a PC.

They suppose to blow the air from inside the case to outside ?
or vice versa ?

How the air flow must move inside the case ?

For example, in my pc there is a fan on the side of the case, parallel
with the motherboard, and it blows the air from inside the case to outside.
Is this the correct way ?

What about the fans to the back of the case. For example the fan inside
the power supply, or any other fan at the back of the case.

b.t.w. is there any way to see/visualize how the air moves inside the case ?

And does it really make significant difference in temperature by adding fans,
or changing their position ?