The first 2 are mainly various electrical and electromechanical equipment. Omron feels like it's mainly small electronic equipment. Siemens produces a lot of categories, but the table equipment, complete systems are available, but now the benefits are not good. The last one is not clear, seems to be made of electrical connectors.
abb: /
Schneider:/schneider/index.jsp
Omron:/
Siemens:/zh/
Electrical related to them, they are all involved in the field will not be different because of different areas. The principle of power supply and distribution is the same for chemical, machinery, metallurgy and so on.