Boss, Manager, Senior
Can I have several bosses in one company? What's the difference between boss and president or CEO?
2. Manager, Senior
What's the difference between manager and senior? When you say senior, that means someone is older than you in the company, right? Whether someone is a senior or not is dependent on the experience, not on the position, right?
1. A boss usually refers to someone who is of a higher position than you and gives you direct orders. Someone you go to when you need approval. It's not the name of a position like president or CEO. President and CEO are titles.
It's quite possible to have more than 1 boss but it typically points to 1 person. You may refer to someone who gives orders to your boss, a boss's boss, or if they have an official position, you refer to them by their position.
Simply speaking, your boss could be the president or CEO, but they could also be much lower in position depending on where you are in that company.
2. Like you said, a manager is the name of the position in a company, a senior is just someone who is older. A Senior manager would be someone who is of a higher position than a manager. You can't use Senior alone as a position name, as it just means they're old. You can call someone manager, but not senior. It would need to be used with another word, or else it would just be rude. There are other usages for "senior" but in a company, it's used more as an adjective than a noun.
Hope that makes sense.