The recovery tool of LineageOS is the TWRP recovery. You boot into it by pressing Volume Up when starting or rebooting the phone or by selecting Recovery in the reboot dialogue of LineageOS. You can make a backup of the current state of the OS there, but beware: It doesn’t handle your data like pictures, contacts, ringtones etc. … you need another way to backup those.
If you followed the migration instructions, you flashed addonsu along the way to enable root. This roots the official LineageOS in principle, but
make sure Root access is enabled for Apps (or Apps and ADB) in Settings - Developer settings.
If you don’t see Developer Settings (I forgot whether it is visible by default), go to Settings - About phone first and continuously tap on Build number until it says you are developer now, Developer settings will be visible afterwards.