Disclaimer: I am a learner, not a native speaker.
Simply put, this particular part of the tutorial confuses the concept of "passive" (피동) with another concept, "causative" (사동).
To give you more details, I will try to discuss this question from the following three aspects: First, the conceptual difference between passive and causative. Second, how Korean forms passive and causative. Third, why word pairs like 끝나다 -> 끝내다 should be classified as causative, rather than passive.
First, let's illustrate the difference between passive and causative with an example. Consider the verb "먹다" (to eat). The passive form of "먹다" is "먹히다" (to be eaten up). So you can say
동생이 밥을 먹어요. My brother is eating the meal. (1)
밥이 동생에게 먹혀요. The meal is eaten by my brother. (2)
Conceptually this is the same as in English, to eat -> eaten.
Now consider another verb: "먹이다". This is the so-called "causative" form of "먹다". It can be translated as "to let (somebody) eat". For example,
엄마가 동생에게 밥을 먹여요. Mom is letting my brother eat (say, feeding him, or telling him to eat). (3)
Note that, the particle "에게" is used in both (2) and (3). In a passive construction like (2), the English "by X" is expressed in Korean as X에게. In a causative construction like (3), the English "let X" is expressed in Korean as X에게. But there's a catch: if the original verb is intransitive, the "let X" part will be expressed, not as X에게, but as X을/를. For example:
동생이 울어요. The brother is crying. (4) (active)
누나가 동생을 울려요. The sister lets (makes) her brother cry. (5) (causative)
This will become important later.
Second, let's review the different ways of forming passive and causative in Korean:
For passive, you have "-이다", "-히다", "-리다", "-기다", "-되다", "-받다", "-당하다", "-어지다" and "-개 되다".
For causative, you have "-이다", "-히다", "-리다", "-기다", "-우다", "-구다", "-추다", "-이우다", "-시키다" and "-게 하다".
Here you can see that "-이다", "-히다", "-리다", "-기다" exist in both the passive and the causative, and this may lead to some confusion.
(These forms are taken from the book <재미있는 국어 문법> published by 천재교육 in 2019. I believe this book is used as a reference book for Korean middle school.)
Finally, let's consider the pair of verbs 끝나다 -> 끝내다 with the help of the following example sentences:
수업이 끝났다. The lesson is over. (6)
선생님이 수업을 끝냈다. The teacher finished the lesson. (7)
You might think that (6) is a passive form of (7) but this is not correct, as there is no way to go from (7) to (6) by adding any of the endings listed above. It can only go the other way around:
끝내다 = 끝나- + -이다,
and -이다 here can only be explained as a causative ending, as the teacher "lets" the lesson to come to an end.
One can also look at this in another way: if (6) were the passive of (7), then the agent would be expressible as "X에게". But
수업이 선생님에게 끝났다.
is a weird, perhaps not grammatical, sentence. However, if you think of (7) as the causative of (6), the 수업을 part just corresponds to the 동생을 part in (5). This is exactly how you express "let X" in a causative construction.
Therefore, in conclusion, 끝나다 is not a passive form of 끝내다. Rather, we should think of 끝내다 as the causative form of 끝나다.
Added: As for your first 3 examples,
그것이 기억나요. literally, that thing comes (to mind). active.
땀이 나요. literally, sweat comes out. active.
화가 났어요. literally, fire came out. active.
