Installing lineageOS
Samsung devices does not have fastboot mode, at least i am not aware of Samsung devices with Mediatek chipset  

the bootloader lock status can be checked only from download mode screen  

step by step instructions vary on device and android version. your android version 5.02 may not have OEM unlocking in developer options, you can skip this step  

on the other hand be aware Samsung does improve security continuously and may roll-out new features even to old devices, which leads in non-working tutorials  

also rooting apps like Magisk do silently change usage at will which may end up in data loss if tutorial isn't up to date