Hello,

Aerospace Engineering is a branch where as avionics is a domain in Aerospace. It is simple as Mathematics is a subject and trigonometry is chapter in mathematics.

Aerospace is a field which provides knowledge and skills for design and development of aircraft,Spacecrafts,missiles. It contains two divisions. one is Aeronautical Engineering and astronautical engineering.

Avionics deals with the hardware part or the electronics part of spacecraft or aircrafts. This is related to many aspects like making a craft automated, increase the performance. It solves problems related to  landing system,Flight safety system,auto-pilot system and many more.

For more details you can read an article on aerospace engineering:- https://www.careers360.com/aerospace-engineering-course

and  on avionics related you can read from the official site of IST:- http://www.ist.edu.pk/aero/about-avionics-engineering

Hope it helps!