Hi,
Making a smartphone requires engineers from different fields and these are the roles of engineers in making a smartphone
System design engineering- to built , choose or edit the architecture
Electronics engineering  - to implement design and dealing with electronics board and circuit .
Electrical engineering - it includes power unit , battery levels , systems
Software engineering- os installation , app development,  app maintenance,  writing code
There are many more fields and if you are interested you can choose any field of your interest.