grammar - How does the passive function in this sentence

The sentence is an example taken from the grammar section of my textbook. 先生が手紙を書いてくださったおかげで、大きい病院で研修を受けられることになった。 => "Because the teacher kindly sent a letter, it happened that I could absolve an internship in a big hospital."

In the section where this sentence comes from "...おかげで/おかげだ" is explained and how it puts the positive outcome X into causal relation to Y. However, this shall not be the focus of the question here. What confuses me here is the use of the bold part 研修を受けられる.

Just before I also learnt about various new applications for the passive mood in japanese. I must say now that this part wasn't very well done by my textbook since it felt like it would mix up some grammatical categories or at least it didn't explain its point well. In this section, I was taught two new ways of using the passive mood:

1) indirect passive (intransitive verbs) 2) indirect passive (transitive verbs)

These are the headings for these two subsections used in the textbook. Now, I didn't really understand these two subsections. One reason therefore is that my textbook showed me examples for

1) indirect passive (intransitive verbs)

with transitive verbs Oo Here's another example: 部長は私に仕事を頼んだ。-> 私は部長に仕事を頼まれた。 According to jisho, 頼む is a transtive verb. Did I understand something wrong about the category "intransitive" here? Do I need to understand this category in a different way here?

Furthermore, the translations for these examples didn't make any effort to transport the semantical difference between a "normal" sentence and these passive constructions. So, while I can definitely still translate and understand such sentences, the nuances brought in by these constructions are lost to me :(

Answer

This 受けられる is the potential form of 受ける. Passive voice is not used in this sentence.

れる/られる has four major meanings. See: Passive usage of 「済まされない」 in sentences

Regarding the usage of を, 研修が受けられる and 研修を受けられる are interchangeable here. See: The difference between が and を with the potential form of a verb.

(This potential れる/られる is a more basic topic than ～おかげで, so I personally don't think the textbook is bad.)