Scratch happen due to tiny dust particles not because of key or knife which youtuber shows because iron and other metals have hardness lower than Gorilla Glass which is 6 but dust particles has hardness of 9 so they will easily scratch any kind of Gorilla Glass whether it's GG6 or GG3. Only sapphire glass has hardness equal to dust particles that is 9. That's why they are used in expensive watches but it is too expensive even apple doesn't use it except on camera lenses. For proof you can watch "Jerry rig everything's" video on poco f1 where he scratches its screen with different grades of hardened materials it gets scratched on level 6 like every other smartphone (even note 9, Apple 10s also gets scratched on level 6). The one who has greater hardness than other will scratch it. Hardness is measured on scale of 1 to 9 where 9 is the hardest material on earth which is Dimond. This is why Dimond is very difficult to cut but it can cut everything. Iron has greater hardness than wood that's why it is able to cut it.
So if there's any dust particle on your screen and you just slides your fingers on it or just rubs your screen against anything like in pocket then it's done. They are so tiny that you can't even notice them. They are everywhere in your pocket, on your fingers everywhere.
The deference between GG3 and GG6 is of only thickness. GG3 is two times thicker than GG6. You can experience this by looking at your screen, you will notice that screen is not touching the glass and there is slight gap between glass and screen because glass has some thickness but if you have ever used GG6 glass phone then you will feel like you are just touching and typing on the screen itself (which feels amazing, like in Samsung Galaxy s9 or note 9) there is no difference between screen and glass, It's that thin but also having same hardness and strength as GG3.
If you hate tempered glass like me but also don't want scratches at the same time then you can use film like screen guards. They don't add any thickness plus doesn't reduce any touch response and don't get bubbles on edges. They are also very clear, you will forget that you have any kind of screen guard on. Here is the link of clear screen guard that I'm using its one of the best film type screen guard available bit expensive but worth it. You can search on youtube "ringke dual easy" to learn how to apply it cause it has four layer and bit tricky. It is self dust removing type of screen guard. If you get bubbles after applying it then don't worry they will vanish after 2 or 3 days. Trust me I'm not promoting anything I'm using this product. There are two screen guard in ane pack. Hope i helped you.
https://www.amazon.in/gp/aw/d/B07JGZR4NH?psc=1&ref=yo_pop_mb_yo_pop_mb_pd_t2
