In summer, should attics simply have fans or other ventilation?
We paid for a solar powered attic fan. It installs near the top of the roof peak and ideally points south (ours is east) and when the attic temperature exceeds 85°F, the fan starts. This presumes that there is sufficient sunshine to power the motor.

Our air conditioning use dropped substantially after the installation. It is still quite hot in the summer in the attic, but not body-killing hot as it usually is.

Because heat rises, you'd want a fan that blows out at the top and pulls cooler air in from below.