what are the main differnce on differnt company computers?

Personal preference and market budgeting.

Sometimes the casing is different. Different casing designs allow for different airflow.

In the case of laptops, you can see more variety in the casing, some with better cooling than others. The motherboards are also different in different brands/models of laptops. This is usually limited to the shape, but sometimes the simple shape makes a difference.

Bundled software. They always bundle their own software with their computer.

Bottom line - When you are looking at a computer, remember that someone else could build a computer with similar parts, and possibly have a more sturdy build overall, and still keep it cheap. Shop around, find something you like.