Learn Russian: Мой брат обязательно будет учиться в университете. - My brother will definitely study at the university.

This is the compound future tense. We take the future form of the verb to be (быть) and add the dictionary form (infinitive) of the main verb. Because my brother is a he, we use the he/she/it form будет along with учиться (to study).

Why do we use учиться here instead of изучать?

We use учиться when talking about studying in general or being a student at a location, like a university or school. You would only use изучать if you were mentioning the specific subject he is studying, like mathematics or Russian.

Why does учиться have a soft sign before the -ся ending?

Because it is in the infinitive form (meaning 'to study'). In the compound future tense, the main verb is always the infinitive. Remember that the infinitive ends in -ться with a soft sign, while the present tense form учится ('he studies') drops it.

I haven't seen обязательно before. How is it used?

It is a very common adverb meaning definitely, certainly, or without fail. You will often hear it when someone is promising to do something or, like in this sentence, expressing strong confidence that an event will happen.

Why does university end with an -е in ?