Someone very knoweldgable please teach me open source

goal
trying to find out how to take open source code
and make it into a working app/software

info
simplenote was made open source https://simplenote.com/2016/08/11/ios-android-and-macos-apps-now-open-source/

things i dont understand + questions

  1. i dont understand why nobody has made another simplenote, it is so well designed, instead ppl are making all these other apps that are so poorly designed. why havent ppl copy/paste this open source code and made another simplenote?

  2. longterm i want to make another simplenote (but does that have a login). i want to know what are all the specific topics i’d need to know to do this ?

  3. what in world is this https://simperium.com/overview/ - is it needed? or?

  4. why did they make this open source, very confused cos wordpress brought simplenote, i dont understand why they did?

  5. anything else that would be helpful (for goal ofc)

is there anyone on here knowledgae enoguh to answer any of these?

please answer as if i was 4 cos i know nothing about coding. i understand it takes a long time to learn coding

‘but does that have a login’ --> but does not have a login