I have an xbox 360 in my room that I don't use, but can I access netflix, youtube etc from there?
That's fine.
Yes, even better than on the Amazon Fire TV.
No. Alone because the Xbox 360 plays a maximum of 720p, the FireTV goes up to 4k.
Also wrong, the Xbox 360 can also play 1080p. Only the games are displayed in 720p natively. However, films and YouTube are possible in FHD.
Jo, it can output 1080p technically, but the apps "Prime Video" and "Netflix" are limited to 720p. Your television always shows you the 1080, but Prime Video / Netflix is at most an "inflated" 720p.
I was also wondering why you set such a limit at 360, but unfortunately it is. You can also inquire at Amazon and Netflix.