I've done homebrew material for 5e before, and in that case the wizard's of the coast license is added to the material.
This is for every material that use the dnd system, as the material is addressed and protected by such license.
So if you want to make an rpg system, how do you do that? How do you, or even can you, make yourself a license? How do other people use your system to make adventures? How to you protect the publication from other companies copying and selling your system?