how does samsung android work

i have xperia Z1 and it was easy to deal with sony android system once i found out that i should unlock the bootloader (nobody told that i should do that ) i asked how can i do that ( didn't get the answer again) but the official way was the easiest
and because i have followed the official way i had to use ADB
after while i understood that there is 3 thing in android
1st level - the system we normally use with calling and playing and browsing
2nd level - recovery in case we wanted to the change some in the 1st level
3rd level - which is the fastboot mode we use it to change recovery and in some case to flash official stock system
that how it work on Sony - nexus - HTC was close enough (have Hboot of instead of fastboot ) but in anyway it's not much of a big deal it's simple
but when it comes to samsung there is the download mode but in this case download mode doesn't work on ADB
there is once when i saw a fastboot mode but entering it was mystery and after i was wondering if the phone wasn't a copy
(not official) it was samsung core prime
but in anyway i couldn't understand how it work
is the fastboot mode on samsung = download mode ?
or does samsung work on 4 levels ?
please explain it to me
sorry for the bad english :3

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