Actually, pretty easily, it turns out. Just because they had ‘primative’ materials to work with doesnt mean they were stupid. While the colossal distance of time means we are uncertain of which methods they used, we can come up with many techniques, using materials and information available to them at the time, that would have worked surprisingly well.
Pyramids? Lots of slaves, wooden rollers, and earthen ramps. Stones ‘too heavy to lift’ were merely pushed up on top of boxes full of sand, with the sand then poured out to lower the blocks into position.
Easter Island? Ditto.
Stonehenge? Probably the same, though some folks think they wrapped the stones in basketlike holders and rolled them. Aligning something to the positions of the stars isn’t hard, especially when they probably already had smaller versions made of wood. Predicting the solstice and equinox is easy when you can work on it for a couple generations.