adb and fastboot help needed

[jury68000]

I'm on Linux Mint 18.3 and my G620S-L01 phone is on Android 4.4.4
I have downloaded minimal adb and fastboot tool.
First question, when issueing "adb devices" and "fastboot devices" command, shouldn't I get the same output? ( which is from what I understand the device serial number ). This is what I get:

./adb devices
List of devices attached
84dbac441b79	device

./adb reboot-bootloader

./fastboot devices
no permissions (user in plugdev group; are your udev rules wrong?); see [developer.android.com/tools/device.html]	fastboot
sudo ./fastboot devices
3a226eb	fastboot

Then when I issue any oem commands I tried, I always get "FAILED (remote: 'unknown command')" like for example:

sudo ./fastboot oem device-info                                                   
FAILED (remote: 'Invalid command')
fastboot: error: Command failed

I have read in some other thread, that to make oem commands working "OEM unlock" needs to be set up in Developer options. But from what I understand, this option is for >= Android 5.something. How to make those commands working on 4.4.4 version?

Edit:
I have then downloaded and installed full-blown Android studio. But the result is the same.