What kind of modifications do you make in automotive mechanics?

When you talk about "modifications", I assume you mean making a car different from the configuration it had when it originally left the factory. And I assume you mean apart from appearance items, color, etc.. Well, there are many, many ways a car can be customized in functional aspects. A popular modification is in the engine itself. Many engine blocks, in fact, are of the same external dimensions but have different displacements or other configurations internally. Folks make this swap to increase power usually. A common block is the GM "small block". There are also modifications folks have made in the frame and engine compartment to accommodate larger engines. Or change the fuel from, say, gasoline to natural gas. Or even more exotic changes.

The suspension is another candidate for modification. Most often, the changes are in making the suspension stiffer and more resilient in order to increase the car's "nimbleness", or ability to absorb punishment and respond more quickly to steering changes. Suspension changes also include changing the brakes, such as larger or more resilient rotors and calipers.

Then we get to even more exotic mods, such as changing -- like the aforementioned natural-gas mod -- to electric. This is a bit harder because it entails major changes not only to the drive components, but to install the batteries themselves, which also tend to be rather expensive.

Some folks have changed the interior of the vehicle, such as to install more form-fitting seats for a secure feeling when pushing the vehicle through maneuvers.

These are the major changes I've done, but I leave cosmetic changes and, say, entertainment systems, to others, since these are extremely varied and their value is even more subjective. Not to say there haven't been some really beautiful mods done.

You can make all modifications, depends on what type of mechanic you are