As far as I'm aware, your only option is to decompile the .jar and cope with the fact that the results are obfuscated, just like everybody else. In the past, there were tools to help deobfuscate the decompiled Minecraft code, as discussed herediscussed here. I have no idea if such tools are still maintained, and as such can't make recommendations.
In theory you could obtain a source license from Microsoft, and get the raw source that way, but only if they were even willing to entertain such a deal in general and with you in the specific.
