What causes the car to understeer and oversteer

Get a friend. have him/ her move the wheel left right until the "play" in the wheel starts to move something. You get down and LOOK.

Look at the steering column and steering coupler. As the column shaft moves the coupler should follow exactly with no lost movement

Move to the pitman arm, look at it along with the column shaft. There should be almost no lost movement there, either

Move to the end of the pitman arm. Watch the arm vs the ball end. Should be ZERO detectable play........if the long end of the pitman moves, the ball end should follow.

Move to the idler arm. You should detect almost no or no movement up and down as it starts to move left/ right.

Move to the outer / inner tie rod ends. Same deal, the "rod" part of the thing moves, the "nut" on the ball end should follow with no detectable play.

Look for movement of the steering gearbox housing in the frame. Should be ZERO

Getting the idea? Take a specific piece that moves, and verify that it moves as it should, with no play if it should not have any, and so on