I can't get OptiFine HD to open its installer. Every time I try, the file just gets unzipped. Necessary details: MacOS Catalina. I tried the Magic Launcher method, but Magic Launcher does not have a new enough version for Minecraft 1.16.3 and when I tried anyway MacOS responded with "“MagicLauncher.app” needs to be updated". I am not sure what to do. I would rather not try Forge because I have done it before and after a year of use eventually broke the entire .jar file. Luckily, I was able get the Minecraft folder out before deleting and reinstalling, so no worlds lost.
My only guess would be that I do not have Java installed. The Java Version of Minecraft does not need Java installed as it has its own Virtual Java.
That seems to be exactly the problem that you are facing.
Either you do not have Java installed at all or you are not using Java to run the Jar file.