In Forge, I just changed the file I needed (for example "projects\forge\src\main\java\net\minecraft\server\MinecraftServer.java"), compiled, obfuscated, and threw it into the server files. How can I do this with Fabric? In the server file I see the files "a.class", "aaa.class", etc. How do I edit them? Mixins are not suitable for me, I want to change the source code itself.

The entire fabric toolchain is fundamentally designed around using bytecode (usually using Mixins) modification, not source modification. There is no concept of "JarModding" like what you are after. I would strongly recommend making a normal mod with mixins (even on forge), you can do basically everything while being more compatible and portable.

fabric does not support this. mixins allow quite a lot of replacement, and Fabric-ASM mod allows even more cursed implementations but i would advise against what you seem to think is "required"