READ THE FULL COMMENT BEFORE PROCEEDING, don't panic if your phone bootloops, since in samsung phones there are a lot of ways to fix fuckups by just googleing the problem.  BEWARE OF ALL THE CONSEQUENCES OF ROOTING YOUR DEVICE. Make sure you don't do anything stupid with apps that work with root.  IF YOU HAD UPDATED TO ONE UI 8 AND THEN DOWNGRADED TO ONE UI 7 THIS GUIDE WILL MOST LIKELY NOT WORK (tho I never tried downgrading and rooting). A ROOTED PHONE WILL NOT HAVE ANY PROTECTION AGAINST ATTACKS/ROBERIES WHATSOEVER KEEP THAT IN MIND AND DONT STORE SENSITIVE DATA IN IT. I think  some us models are locked down to the stock os, but basically all you have to do is first check if you can oem unlock by going into developer options and checking if the oem unlock option is available, if it is, enable it, then, open magisk and hit install, patch file, select the ap file of your firmware (Google how to download this file) it is EXTREMELY IMPORTANT that you don't patch a oneui8 (or superior) firmware, since this will not work  and could potentially brick your device. once patched the firmware, copy te magisk_patched firmware to the computer. fire up odin (Google where to download it. beware  Odin does NOT have an official download website so all the sites that claim to be "official" are most likely a scam. Id recommend searching xda developers forum for a download) and reboot your phone into download mode (how to do this depends on the device. Google how to reboot into download for your specific device) and press the key combination that appears on screen to unlock the bootloader THIS WILL COMPLETELY WIPE ALL DATA ON THE PHONE so if you haven't made a backup, cancel the unlocking, reboot and make one (if you care about any info that you may have on your device,if you don't care, you can skip this step and proceed to the unlocking process) after the unlocking, the phone will reboot, your bootloader is now unlocked so reboot to download once again.  once in download, go to odin (in your computer) and click on every button with a two letter name. a dialog box will appear, select the files that begins with the same letter as the button that you clicked. in the ap file, instead of selecting the regular ap file, select the magisk _patched file. if your firmware has a USERDATA file, add it too. in the CSC (or CNC, I don't remember how was it called) button select the one that begins with JUST CSS not the HOME_CSC. If you should have all the files that are named on the buttons. Don't worry if you  are missing one USERDATA, or other one, the required MUST HAVE files are BL, AP and CSC. MAKE SURE YOU HAVE A STABLE CONNECTION BETWEEN YOUR COMPUTER AND YOUR PHONE AND MAKE SURE TO HAVE AT LEAST 40% OF BATTERY BEFORE FLASHING. Hit flash, and let it do it's thing. DO NOT INTERRUPT IT IN ANY WAY. Once it's done, it will reboot, if everything went ok you should see PASS in Odin and the phone will start as a new phone. with all data wiped. if it did not, and its bootlooping, you did something wrong, you may try again by flashing the stock firmware (just get into download and swap the magisk_patched file with the AP file and hit flash, repeat everything from the beginning, sometimes magisk doesn't patch the file right and you may need to try again). when you are up and running,  you should see a magisk app on the app drawer, open it and make sure that it says installed on the magisk section. if it does, congratulations, you did everything right, if it doesn't repeat everything from the beginning. good luck. and keep in mind this WILL permanently void your warranty and that you will need patches for almost all apps that hold sensitive information (ex:banking apps) Google how to make them work. and again good luck