if i was hired at a shell and my boss bought an exxon can he make me work at the exxon and is it legal for him to own to bussiiness which are competer

Unless you have a contract limiting the grounds for which you can be fired, or specifying that you will be employed for a certain period of time, you can be fired for any reason not prohibited by statute (e.g. race, religion, gender, etc.)  As for whether your employer can properly own competing franchises, it may be a breach of your employer's agreement with shell and/or exxon, but that is between your employer and the franchisor.