Answer 1

Public free to use for everyone
Private no one can use it but themself
Protected its for you and your children
default(package) everyone in same package/folder

Answer 2

Search bar is here:
https://www.sololearn.com/post/10362/?ref=app
(Tips: Remember to switch filter option to "Most Popular" first)

Answer 3

https://www.sololearn.com/learn/Java/2156/

Click the "Try It Yourself" in the lessons, make some changes, if you don't understand anything, post your code for us to explain