How much(%) would I be able to learn Java if complete sololearn's Java course ? What's next step Afterward?

Question

I am not from Computer Science Background.

Answer 1

40% if you ignore GUI, EE/Spring, android, testing, other frameworks, and non-basic parts of standard api

next you can learn more about generic, lambda & streams, data structures implemented in standard api, io streams, date time api, i18n, networking, concurrency ..

compare language themes:
https://docs.oracle.com/javase/tutorial/java/TOC.html
https://docs.oracle.com/javase/specs/jls/se18/html/index.html

Answer 2

Probably 5%. Sololearn cover the basics bud. There's a LOT more to every language you can learn here. But it's up to you to reach out and find all that.