Lets say I was to follow Brackey’s tower defense tutorials and was to convert the code from the tutorial into code that would work for multiplayer. How difficult would this be? What would be involved? Any tips if this is possible?
You don’t really “convert” a game to multiplayer. The whole process of thinking about the game loop for a multiplayer game is (at least currently) different than that of a single player game due to all of the new moving parts and the multitude of edge cases you have to design into a multiplayer experience.
What people have told me in the times that I’ve asked is “focus on building one or the other”. This applies to the game design, and to your own self-learning journey. I, myself, did a ton of single player proof of concepts for the first 6 months of my Unity learning so that I could get familiar all things Unity, and then I converted all of my time over to focusing on designing multiplayer game experiences and building the game tech around those experiences.