You can go with systematic approach,
1) Complete your academic studies
2) Get familiarized with using all types of smartphones
3) Join a good mobile phone repair training institute and get certified
4) Buy essential tools and some dead phones to practice on them
5) Gain experience
6) Take assistance from internet
Appreciate your question and the option you have chosen to be a technician.
Always the experience and hard-works make the person as expertise . So you plan the path where you wanted to be expertise and invest time towards it. This is not only about trouble fixing on a product as you highlighted dont limit the expertise to one product. Learn completely and go in depth . with those experience you can even do small projects which can automate the things on electronics.
Also we talk about career on this stream i would advise do your degree on electronics then purse your dreams accordingly on engineering,technology path, Keep the sprit always.
Wish you all the very best, Keep in touch with me for further queries .
First thing first most of the information (infact about chipset) about the phones are available youtube and internet. you need to ensure your basics foundation is strong to fix phone related issue, which means soldering, blower soldering using multi-meter. understand components of phones how they function. I would suggest start working for a phone service center to acquire right skill set..