1. He is my boss.

(What is the meaning of 'boss'? My senior in a department or the president of a company?)

Your boss is your supervisor in whatever job you have. Employees obey the boss, who usually has his/her own boss who needs to be obeyed, etc. And yes, your boss could be the president of a company if you work directly for him/her.

Your boss can be any supervisor. She or he is anyone who is responsible for your work.

If your job is to make and serve hamburgers at McDonald's, your bosses are the shift manager, the store manager, and the president of McDonald's.

The term "boss" can refer to someone who holds a position of authority or leadership within an organization. It can have different meanings depending on the context. In the case of "He is my boss," it typically refers to a person in a supervisory or managerial role who is directly responsible for overseeing and directing the work of others. This could be someone who is in charge of a department, a team, or even the entire company. Without further context, it is not clear whether the person being referred to as "boss" is a senior in a department or the president of a company.