"the open source is in use because of the easy way of seeing the codes and working them out"
You know you can look at the Windows source code too via Microsoft? Publically available code might be of marginal assistance to a hacker, but they are able to quite happily find holes in closed source code too. I would also note that available source code doesn't seem to make software more secure as is often claimed - see the recent many years old holes on Open SSL, BASH, etc.
"We all have the Windows phones for best safety."
Don't disagree there, but it's got little to do with availability of the source code imo...