Android Oreo question

[J.b091]

I have Android Oreo 8.1.0 in my phone & I'm wondering if apps are automatically killed when for example I turn off screen. I'm asking this because I noticed in "recent apps" window there are some apps opened like gallery, camera, messenger and etc... But if I will go in settings/installed apps and check which apps are running I see that those apps aren't running, but different ones like for example google duo, facebook, bluetooth (even if it's disabled), themes & etc... If I remember correctly before when I had android kitkat those apps in recent apps where actually also running in background. Did Oreo fixed it? Also why some apps are running I don't want, for example this google duo or bluetooth?