I'm new so I hope I am posting this in the right place.
From the book(s) I am learning from I have learned that 'aru' should be used instead of 'iru' in the -te iru form when the verb is (or is used as) a transitive verb.
But I think I remember seeing 'miru' used with 'iru' and not 'aru' in this form, even though it is transitive... So now I am pretty confused concerning the relation between a verb being transitive/intransitive and using -te iru/aru form.
Pre-answer Edit: I am starting to assume that 'aru' actually adds a certain nuance to the meaning in the -te iru form and is not actually required in all cases for transitive verbs, but I am still confused as to when exactly I should use -te aru and not -te iru.
Have you already learned that the function of the -te iru form differs depending on the type of verbs? The -te iru form of durative verbs (継続動詞) is an on-going action, i.e., it's the present progressive tense (e.g. 走っている, 食べている), whereas the -te iru form of punctual verbs (瞬間動詞) expresses the present state resulting from the past action (e.g. 結婚している, 死んでいる).
The -te aru from is used to express the present state resulting from the past action of
durative verbs. In transitive-intrasitive pair verbs, transitive verbs are mostly durative verbs, and intransitive verbs are mostly punctual verbs, so you can use two different expressions in almost the same meaning.
e.g.
壁に絵が掛かっている (-te iru form of the intransitive verb 掛かる)
壁に絵が掛けてある (-te aru form of the transitive verb 掛ける)
教室に椅子が並んでいる (-te iru form of the intransitive verb 並ぶ)
教室に椅子が並べてある (-te aru form of the transitive verb 並べる)
As for the difference in nuance between these two expressions, the -te aru form of transitive verbs suggests the existence of the agent/doer. (As you can see, the -te aru form is more important for transitive verbs that don't have the intransitive counterpart.)
Incidentally, the agent is not always "someone else". The speaker can be the agent. Also, を can be used to indicate the object, not always が.
e.g.
お風呂
を沸かしてありますから、どうぞ入ってください。
すぐに料理ができるように、材料
を買ってあります。
It's said that を has a nuance that it's done previously for a purpose, comparing to が, so it's close to the -te oku form in meaning.
When 開く is used as an example, it's better to put furigana on the kanji since 開く can be read both as あく(intransitive) and ひらく(transitive/intransitive). ひらいてある is valid, but あいてある is invalid because あく is intransitive.