WARNING: OUTDATED TUTORIAL. I will get around to updating this at some point, but for now I don't recommend following this tutorial to build the apps.
Seen all the tutorials for building RSDKv5(U) on Mac that you can muster but still haven't gotten it up and running yet? Well, I hope you can handle one more, because this will finally be the one to fix it! I can guarantee it because I was in your shoes and had to figure all this stuff out myself. This is a text tutorial for learning how to decompile RSDKv5(U) and run Sonic Mania Plus or Sonic Origins on your Mac.
How to build RSDKv5:
Step 1: Install Xcode and the command tools (the command tools can be installed by running this command: xcode-select --install but only after installing Xcode). If you already have them, move on to step 2.