A person with a degree in history - if it's high enough - can do whatever requires knowledge of his specific field. I've known a couple of middle east historians who used to be special advisers for certain intelligence units in the army. A person with a degree in history can also become a cultural attache in an embassy. Additionally, people who studied history learn critical thinking skills, which can be applied in various situations.

A historian, however, is someone who specifically studies history for a living. They're usually lecturers in collages and institutions, and their job is to research. So other than giving lectures to students, they mostly research and publish articles and books about their chosen field. Just like any other theoretical academic, a historian's job is to widen the pool of knowledge regarding a specific topic, converse with other researchers, and publish conclusions accordingly.

As an additional add on, what does a history undergraduate actually do in college/university? Is it literally pick a topic then read as much about it as possible with reliable books as sources then write an exam or something more?

Historian here. Today I taught two classes at one of the three institutions of higher learning that pay my bills. The first was my global environmental history class, and today's lesson was part of a multi-part unit on the history of science. I lectured briefly on James Scott's ideas of local and universal knowledge, then the class read some passages from Darwin. I'm pushing them toward the idea that Darwin's theories of evolution work to "naturalize" competition and market relationships that sanction the intensification of resource use and commodify "nature"; science and the market thus combine as forms of universal knowledge that overwhelm more local forms of knowledge that stress the particulars of species and landscapes.

My second class was Western Civilization part I, and today's lesson continued a unit on the development of commerce and cities in the centuries after about 1000. We read a wonderful series of documents detailing the creation of local government in Ipswich in the year 1200, and discussed the combination of coercion and consent these sources documented. On Thursday we're discussing the Life of Cola de Rienzo, a tale of radical politics in Rome in the 14th century.

I wrote some emails, met a few students in office hours, and put off grading a big stack of papers. I'll round out the evening with a bit of work related to my research on the history of bread by reading some of Henry Mayhew's accounts of London workers in the 1850s.

This was pretty typical of a day of work during the school year. At other times of the year I'll concentrate much more on writing, and sometimes traveling to work at archives--not this summer though, this summer is all writing.

the closest occupation to "just historian" would be either an academic professor of history (who's job is education of students and writing/research) or writer-historian (more pop-hist version fo the former, but there is a small overlap).

People use the term "historian" rather loosely, so it kind of depends on your personal feelings on the word. Technically, if you simply like history and read several books on a historical topic and maybe watched a documentary or two, then you could call yourself a "historian." If you "dabble" in history-writing, follow up historical topics on the side, you could call yourself "historian."

But if you mean "historian" as a profession, then that's different. If you mean someone who not only studied history in college but went on to graduate school at the PhD level in history, learning the historiography of the field, the methods and theories of history, doing archival research and field work, writing a dissertation, graduating with a history PhD, and (for the privileged few) finally embarking on an academic career as a tenure-track history professor at a college or university, then obviously calling someone like that a "historian" would seem too expansive and all-encompassing to be meaningless. Academic historians do call themselves "historians" sometimes but in an informal sense.

TL;DR: All history professors are historians. Not all historians are history professors. Not all "historians" are trained in the rigorous level of graduate school. I suppose you could call them "popular" historians or "amateur" historians. Much like we have "amateur" sleuths, "amateur" scientists, and so forth.

Well, I chose to teach with my degrees. But I also answer questions on ask historians. Lol.