Aero: Self explanatory
Transmission: Affects how much traction you get when accelerating and decelerating. It also plays a big roll in tyre wear. Increasing diff will give you tyre wear and more outright traction while lower diff will give you less tyre wear with more gradually traction
Suspension geometry: Increasing Camber will increase grip through corners. Too much camber will wear increase tyre wear. Decreasing camber will obviously do the opposite. Increasing toe will give more responsiveness on turn in and help with straight line stability. Too much toe will also increase tyre wear
Suspension: Increasing front or rear suspension stiffness helps with hard braking and accelerating. Too stiff and the car will feel skittish and bad on tyres. Too soft and you’ll suffer with stability in braking and accelerating. Increasing anti roll bars will help with turn in response but too much negatively affects tyre wear. Decreasing helps with traction through long corners but have a little less turn in response. Increasing ride height gives you more drag and less cornering stability. Try to get the car as low as possible without bottoming out. If you looking back and see sparks, you’re bottoming out.
Brakes: Increase braking pressure: Slow down quicker, easier to lock up. Decrease brake pressure: Slow down slower, lock up less. Brake bias: If you’re locking up the fronts, move bias back. If you are locking up the rears, move is forward. General rule of thumb: More brake pressure, more rearward bias
Tyre pressure: Softer tyres give you more traction with less response. Harder tyres will give less traction and more turn in response but increase tyre temps, which leads to overheating which leads to more tyre wear and less grip
Weight distribution: More ballast: more oversteer. Less ballast: more understeer
If you lookup a setup ie: f1 2018 Australia setup, there is a website called realsport101 and it gives you a good recommended setup. Then you can adjust the setup to suit you from there. I would stay away from the YouTube setup guides because they are generally for hot lapping and not for racing.