Installing TWRP

Hello there!
I am watching this tutorial (How to Install TWRP & Root w/ Magisk on Samsung Android! [UNIVERSAL METHOD] [Android Root 101 #2] - YouTube) by Dr.E and trying to root my phone. In the video(14:32) Max Lee said that it is time to download a custom ROM. Is it necessary or can I skip it?? I really don’t know what it is and I am supposed to download.

No you dont need to download a custom rom when you just want to root your stock android rom.