They are all correct and normal.
1. You did a good job. << past simple tense
2. You do a good job. << present simple tense
3. I've learned something. << present perfect tense
4. I learn so much. << present simple tense
No. 2 is present simple tense which indicates a generality. It is more likely to be heard as "He does a good job", but "You do a good job" is certainly correct and possible.
Why don't you get XYZ to fix your car? They do a good job.
Last time my car needed servicing I took it to XYZ. They did a good job.
You should try to get a contract with the council. After all, you do a good job. I'm sure they'd be delighted to give you a contract.