Does Time Machine backup everything?

I want to backup my mac because it needs repairing, If I create a Time Machine backup and then my data gets deleted, will I be able to use the backup to just pick up where I left off? Will everything be saved e.g. will plugins for Logic Pro X need to be reinstalled or will they work just as before? will my saved settings for Logic, Synth Presets etc. be exactly where they were before?

Yes. There are some exceptions, but generally the answer is yes. Here’s the rundown: