It depends on your HR policies. You might need to disclose more to HR than to your boss if you are applying for FMLA. Without FMLA, you job is not protected. Then, you can only take leave if your company agrees and they do not have to hold your job open for you if you want to return. For FMLA, you will likely need documentation from a doctor.

That depends on your specific employer.

Do I have to give a reason for an unpaid leave of absence? If so, how ‘thorough’ does it have to be?