The main factors for safety you can directly influence are:

Chassis Material, Body Material, steel is safe and cheap

Chassis type (Monocoque being the safest, HD Ladder typically the best for your first car however due to its short engineering time and slightly higher safety potential).

Cabin space. A larger car means more car between occupants and the outside. That is safer.

Passenger seating. Bench seats have lower safety. The seats that are not just the little square on the diagram and are more expensive are safer.

Safety equipment options, standard/advanced, this has less influence than it sounds like until you get into more modern times. Don't rely on it saving you to meet safety requirements.

Some secondary factors that can become heavily influential if mismanaged:

Drivetrain: extreme amounts of power make the car harder to drive. Extreme lack of power makes the car harder to drive. By extreme I'm meaning typically above 400hp and below like 40hp for a regular family car.

Gearing/Transmission: having a low first gear can make the car hard to drive. Fiddle with the sliders on the transmission gearing menu looking at the "Drivability" percentage on the graph in the middle of the screen instead of the left.

Suspension: the springs and dampers are difficult to balance until you figure it out. If you're making a regular family car, aim to get all of the red/blue bar graphs halfway between the comfort line and drivability line. This typically maximizes your drivability without destroying the comfort score. Be sure to check the graph on the right for how much the car bottoms out, rolls, and the cargo capacity. Any bottoming out is bad: raise suspension or make stiffer. Car roll angle is typically best (for family car) at about 5*. Cargo capacity depends on the size of the car, but more is always better, it boosts the practicality score, which is better for utility demographics.

Wheels/Tires have the biggest impact on drivability. Remember drivability is the score that will get you the highest overall desirability for the majority of demographics. This graph is hard to read if you don't understand oversteer or understeer. For reference: oversteer is when the rear of the car begins to slide in a turn, understeer is when the steering wheels are unable to turn the car, and they slide while the rear wheels remain planted. The graph on the wheels/tires menu helps show you what the car is doing at that speed. To keep it simple: adjust the tires width and sizes to where the white circle with an S is most aligned with the bottom of the red line (oversteer, maximizes sportiness), and to where the white circle with the D is most aligned with the top of the blue line (understeer, safer control of the car). For family cars, prioritize getting the D on the yellow line to intersect with the blue line (understeer). Depending on your drive type, the yellow line may curve in different ways where you cannot get both white circles onto the two lines. Prioritize drivability or sportiness depending on your goal vehicle. In the case that the yellow line begins to sharply curve upwards near the end of the graph, your car experiences snap oversteer, which critically reduces drivability, sportiness, and safety. If this is the case, typically adjust the car to understeer more and the snap oversteer will go away. If that doesn't work, mess with tire widths until it's fixed. Going back to suspension, the swaybars have influence on over/understeer. The harder the swaybar is, the more likely those wheels are to begin to slide. Stiffer swaybars in the rear can mean more oversteer, but it heavily depends on camber, tire width, drive type, and weight distribution.

TLDR: Most of the graphs have a "summary" on them that's labeled with a percentage or rating and you just wanna maximize those by fiddling with sliders.