What causes the car to understeer and oversteer

Thanks for all of the replies, I just got this car so haven’t really had much of s chance to play around with it yet.
I took it to good year tire, the told me everything in the suspension had been replaced except for the Upper and lower control arm bushings so I paid them to do that.
They also showed me while the car was on the rack that idler arm was loose they tightened that.
The Mech also told me that the steering box was a rebuilt, found out this morning that the Power steering pump was empty?? Lots of surprises