What should one do if he has completed sololearn and wants to design complex applications.

Answer 1

By trying to make one. I see you haven't posted any codes yet. Try to make something yourself. Try solving some coding challenges.

Answer 2

do some challeges and make your own code. if you feel like you don’t know enough for complex designs then you may need to find other means of studying as sololearn is pretty basic with their courses but theres also plenty of lessons on here you could search for