If you want to open your own shop, I suggest learning from a good master is the most realistic. What you learned from school is only a little stronger in theory. Nowadays, repairing mobile phones depends on practical experience.
At present, the profit of opening a mobile phone shop is still ok. The manufacturer's warranty for one year may not have much advantage for you to open a shop. If every mobile phone with quality problems is returned to the manufacturer for repair, you have to calculate how much the cost will be higher, and the manufacturer will not be able to return it for a week at the earliest. If it is a month late, will the customer have to wait? You can solve small problems yourself, and customers will solve them immediately. After a long time, you are also the most advantageous competitor in the local area!
I wish you an early success! You can consult me if you have any questions, and I will try my best to help you solve the technical problems!