What is the best way to deepen our knowledge after getting the certificate?

Answer 1

assign yourself some tasks and practice ..
you could also get practice questions on the internet.

Answer 2

as i always say, imagine a project, try to code in any language you have learned and ask for help wherever you find difficulties. For example i made a program which calculates what is my shift (yeah i work on shifts) for the next 50 years(!!!) in 4 languages and it is actually very helpful to me. I can even run it through sololearn's playground!

Answer 3

Practice makes perfect.